d56239a82e
16 Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
Anna-Maria Behnsen
|
f004bf9de0 |
timers/migration: Fix grammar in comment
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-8-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
2367e28e23 |
timers/migration: Spare write when nothing changed
The wakeup value is written unconditionally in tmigr_cpu_new_timer(). When there was no new next timer expiry that needs to be propagated, then the value that was read before is written. This is not required. Move the write to the place where wakeup value is changed changed. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-7-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
835a9a67f5 |
timers/migration: Rename childmask by groupmask to make naming more obvious
childmask in the group reflects the mask that is required to 'reference' this group in the parent. When reading childmask, this might be confusing, as this suggests, that this is the mask of the child of the group. Clarify this by renaming childmask in the tmigr_group and tmc_group by groupmask. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-6-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
d47be58984 |
timers/migration: Read childmask and parent pointer in a single place
Reading the childmask and parent pointer is required when propagating changes through the hierarchy. At the moment this reads are spread all over the place which makes it harder to follow. Move those reads to a single place to keep code clean. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-5-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
3ba111032b |
timers/migration: Use a single struct for hierarchy walk data
Two different structs are defined for propagating data from one to another level when walking the hierarchy. Several struct members exist in both structs which makes generalization harder. Merge those two structs into a single one and use it directly in walk_groups() and the corresponding function pointers instead of introducing pointer casting all over the place. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-4-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
9250674152 |
timers/migration: Improve tracing
Trace points of inactive and active propagation are located at the end of the related functions. The interesting information of those trace points is the updated group state. When trace points are not located directly at the place where group state changed, order of trace points in traces could be confusing. Move inactive and active propagation trace points directly after update of group state values. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-3-757baa7803fe@linutronix.de |
||
|
Anna-Maria Behnsen
|
10a0e6f3d3 |
timers/migration: Move hierarchy setup into cpuhotplug prepare callback
When a CPU comes online the first time, it is possible that a new top level
group will be created. In general all propagation is done from the bottom
to top. This minimizes complexity and prevents possible races. But when a
new top level group is created, the formely top level group needs to be
connected to the new level. This is the only time, when the direction to
propagate changes is changed: the changes are propagated from top (new top
level group) to bottom (formerly top level group).
This introduces two races (see (A) and (B)) as reported by Frederic:
(A) This race happens, when marking the formely top level group as active,
but the last active CPU of the formerly top level group goes idle. Then
it's likely that formerly group is no longer active, but marked
nevertheless as active in new top level group:
[GRP0:0]
migrator = 0
active = 0
nextevt = KTIME_MAX
/ \
0 1 .. 7
active idle
0) Hierarchy has for now only 8 CPUs and CPU 0 is the only active CPU.
[GRP1:0]
migrator = TMIGR_NONE
active = NONE
nextevt = KTIME_MAX
\
[GRP0:0] [GRP0:1]
migrator = 0 migrator = TMIGR_NONE
active = 0 active = NONE
nextevt = KTIME_MAX nextevt = KTIME_MAX
/ \
0 1 .. 7 8
active idle !online
1) CPU 8 is booting and creates a new group in first level GRP0:1 and
therefore also a new top group GRP1:0. For now the setup code proceeded
only until the connected between GRP0:1 to the new top group. The
connection between CPU8 and GRP0:1 is not yet established and CPU 8 is
still !online.
[GRP1:0]
migrator = TMIGR_NONE
active = NONE
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = 0 migrator = TMIGR_NONE
active = 0 active = NONE
nextevt = KTIME_MAX nextevt = KTIME_MAX
/ \
0 1 .. 7 8
active idle !online
2) Setup code now connects GRP0:0 to GRP1:0 and observes while in
tmigr_connect_child_parent() that GRP0:0 is not TMIGR_NONE. So it
prepares to call tmigr_active_up() on it. It hasn't done it yet.
[GRP1:0]
migrator = TMIGR_NONE
active = NONE
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = TMIGR_NONE
active = NONE active = NONE
nextevt = KTIME_MAX nextevt = KTIME_MAX
/ \
0 1 .. 7 8
idle idle !online
3) CPU 0 goes idle. Since GRP0:0->parent has been updated by CPU 8 with
GRP0:0->lock held, CPU 0 observes GRP1:0 after calling
tmigr_update_events() and it propagates the change to the top (no change
there and no wakeup programmed since there is no timer).
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = TMIGR_NONE
active = NONE active = NONE
nextevt = KTIME_MAX nextevt = KTIME_MAX
/ \
0 1 .. 7 8
idle idle !online
4) Now the setup code finally calls tmigr_active_up() to and sets GRP0:0
active in GRP1:0
[GRP1:0]
migrator = GRP0:0
active = GRP0:0, GRP0:1
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = 8
active = NONE active = 8
nextevt = KTIME_MAX nextevt = KTIME_MAX
/ \ |
0 1 .. 7 8
idle idle active
5) Now CPU 8 is connected with GRP0:1 and CPU 8 calls tmigr_active_up() out
of tmigr_cpu_online().
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T8
/ \
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = TMIGR_NONE
active = NONE active = NONE
nextevt = KTIME_MAX nextevt = T8
/ \ |
0 1 .. 7 8
idle idle idle
5) CPU 8 goes idle with a timer T8 and relies on GRP0:0 as the migrator.
But it's not really active, so T8 gets ignored.
--> The update which is done in third step is not noticed by setup code. So
a wrong migrator is set to top level group and a timer could get
ignored.
(B) Reading group->parent and group->childmask when an hierarchy update is
ongoing and reaches the formerly top level group is racy as those values
could be inconsistent. (The notation of migrator and active now slightly
changes in contrast to the above example, as now the childmasks are used.)
[GRP1:0]
migrator = TMIGR_NONE
active = 0x00
nextevt = KTIME_MAX
\
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = TMIGR_NONE
active = 0x00 active = 0x00
nextevt = KTIME_MAX nextevt = KTIME_MAX
childmask= 0 childmask= 1
parent = NULL parent = GRP1:0
/ \
0 1 .. 7 8
idle idle !online
childmask=1
1) Hierarchy has 8 CPUs. CPU 8 is at the moment in the process of onlining
but did not yet connect GRP0:0 to GRP1:0.
[GRP1:0]
migrator = TMIGR_NONE
active = 0x00
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = TMIGR_NONE migrator = TMIGR_NONE
active = 0x00 active = 0x00
nextevt = KTIME_MAX nextevt = KTIME_MAX
childmask= 0 childmask= 1
parent = GRP1:0 parent = GRP1:0
/ \
0 1 .. 7 8
idle idle !online
childmask=1
2) Setup code (running on CPU 8) now connects GRP0:0 to GRP1:0, updates
parent pointer of GRP0:0 and ...
[GRP1:0]
migrator = TMIGR_NONE
active = 0x00
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = 0x01 migrator = TMIGR_NONE
active = 0x01 active = 0x00
nextevt = KTIME_MAX nextevt = KTIME_MAX
childmask= 0 childmask= 1
parent = GRP1:0 parent = GRP1:0
/ \
0 1 .. 7 8
active idle !online
childmask=1
tmigr_walk.childmask = 0
3) ... CPU 0 comes active in the same time. As migrator in GRP0:0 was
TMIGR_NONE, childmask of GRP0:0 is stored in update propagation data
structure tmigr_walk (as update of childmask is not yet
visible/updated). And now ...
[GRP1:0]
migrator = TMIGR_NONE
active = 0x00
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = 0x01 migrator = TMIGR_NONE
active = 0x01 active = 0x00
nextevt = KTIME_MAX nextevt = KTIME_MAX
childmask= 2 childmask= 1
parent = GRP1:0 parent = GRP1:0
/ \
0 1 .. 7 8
active idle !online
childmask=1
tmigr_walk.childmask = 0
4) ... childmask of GRP0:0 is updated by CPU 8 (still part of setup
code).
[GRP1:0]
migrator = 0x00
active = 0x00
nextevt = KTIME_MAX
/ \
[GRP0:0] [GRP0:1]
migrator = 0x01 migrator = TMIGR_NONE
active = 0x01 active = 0x00
nextevt = KTIME_MAX nextevt = KTIME_MAX
childmask= 2 childmask= 1
parent = GRP1:0 parent = GRP1:0
/ \
0 1 .. 7 8
active idle !online
childmask=1
tmigr_walk.childmask = 0
5) CPU 0 sees the connection to GRP1:0 and now propagates active state to
GRP1:0 but with childmask = 0 as stored in propagation data structure.
--> Now GRP1:0 always has a migrator as 0x00 != TMIGR_NONE and for all CPUs
it looks like GRP1:0 is always active.
To prevent those races, the setup of the hierarchy is moved into the
cpuhotplug prepare callback. The prepare callback is not executed by the
CPU which will come online, it is executed by the CPU which prepares
onlining of the other CPU. This CPU is active while it is connecting the
formerly top level to the new one. This prevents from (A) to happen and it
also prevents from any further walk above the formerly top level until that
active CPU becomes inactive, releasing the new ->parent and ->childmask
updates to be visible by any subsequent walk up above the formerly top
level hierarchy. This prevents from (B) to happen. The direction for the
updates is now forced to look like "from bottom to top".
However if the active CPU prevents from tmigr_cpu_(in)active() to walk up
with the update not-or-half visible, nothing prevents walking up to the new
top with a 0 childmask in tmigr_handle_remote_up() or
tmigr_requires_handle_remote_up() if the active CPU doing the prepare is
not the migrator. But then it looks fine because:
* tmigr_check_migrator() should just return false
* The migrator is active and should eventually observe the new childmask
at some point in a future tick.
Split setup functionality of online callback into the cpuhotplug prepare
callback and setup hotplug state. Change init call into early_initcall() to
make sure an already active CPU prepares everything for newly upcoming
CPUs. Reorder the code, that all prepare related functions are close to
each other and online and offline callbacks are also close together.
Fixes:
|
||
|
Anna-Maria Behnsen
|
facd40aa5c |
timers/migration: Do not rely always on group->parent
When reading group->parent without holding the group lock it is racy
against CPUs coming online the first time and thereby creating another
level of the hierarchy. This is not a problem when this value is read once
to decide whether to abort a propagation or not. The worst outcome is an
unnecessary/early CPU wake up. But it is racy when reading it several times
during a single 'action' (like activation, deactivation, checking for
remote timer expiry,...) and relying on the consitency of this value
without holding the lock. This happens at the moment e.g. in
tmigr_inactive_up() which is also calling tmigr_udpate_events(). Code relys
on group->parent not to change during this 'action'.
Update parent struct member description to explain the above only
once. Remove parent pointer checks when they are not mandatory (like update
of data->childmask). Remove a warning, which would be nice but the trigger
of this warning is not reliable and add expand the data structure member
description instead. Expand a comment, why it is safe to rely on parent
pointer here (inside hierarchy update).
Fixes:
|
||
|
Levi Yun
|
d7ad05c86e |
timers/migration: Prevent out of bounds access on failure
When tmigr_setup_groups() fails the level 0 group allocation, then the
cleanup derefences index -1 of the local stack array.
Prevent this by checking the loop condition first.
Fixes:
|
||
|
Anna-Maria Behnsen
|
7a96a84bfb |
timers/migration: Return early on deactivation
Commit |
||
|
Frederic Weisbecker
|
61f7fdf8fd |
timers/migration: Fix ignored event due to missing CPU update
When a group event is updated with its expiry unchanged but a different
CPU, that target change may go unnoticed and the event may be propagated
up with a stale CPU value. The following depicts a scenario that has
been actually observed:
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = TGRP1:0 (T0)
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T0
/ \
0 (T0) 1 (T1)
idle idle
0) The hierarchy has 3 levels. The left part (GRP1:0) is all idle,
including CPU 0 and CPU 1 which have a timer each: T0 and T1. They have
the same expiry value.
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = KTIME_MAX
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T0
/ \
0 (T0) 1 (T1)
idle idle
1) The migrator in GRP1:1 handles remotely T0. The event is dequeued
from the top and T0 executed.
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = KTIME_MAX
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T1
/ \
0 1 (T1)
idle idle
2) The migrator in GRP1:1 fetches the next timer for CPU 0 and finds
none. But it updates the events from its groups, starting with GRP0:0
which now has T1 as its next event. So far so good.
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = KTIME_MAX
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T1
/ \
0 1 (T1)
idle idle
3) The migrator in GRP1:1 proceeds upward and updates the events in
GRP1:0. The child event TGRP0:0 is found queued with the same expiry
as before. And therefore it is left unchanged. However the target CPU
is not the same but that fact is ignored so TGRP0:0 still points to
CPU 0 when it should point to CPU 1.
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = TGRP1:0 (T0)
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T1
/ \
0 1 (T1)
idle idle
4) The propagation has reached the top level and TGRP1:0, having TGRP0:0
as its first event, also wrongly points to CPU 0. TGRP1:0 is added to
the top level group.
[GRP2:0]
migrator = GRP1:1
active = GRP1:1
nextevt = KTIME_MAX
/ \
[GRP1:0] [GRP1:1]
migrator = NONE [...]
active = NONE
nextevt = TGRP0:0 (T0)
/ \
[GRP0:0] [...]
migrator = NONE
active = NONE
nextevt = T1
/ \
0 1 (T1)
idle idle
5) The migrator in GRP1:1 dequeues the next event in top level pointing
to CPU 0. But since it actually doesn't see any real event in CPU 0, it
early returns.
6) T1 is left unhandled until either CPU 0 or CPU 1 wake up.
Some other bad scenario may involve trees with just two levels.
Fix this with unconditionally updating the CPU of the child event before
considering to early return while updating a queued event with an
unchanged expiry value.
Fixes:
|
||
|
Frederic Weisbecker
|
f55acb1e44 |
timers/migration: Fix endless timer requeue after idle interrupts
When a CPU is an idle migrator, but another CPU wakes up before it,
becomes an active migrator and handles the queue, the initial idle
migrator may end up endlessly reprogramming its clockevent, chasing ghost
timers forever such as in the following scenario:
[GRP0:0]
migrator = 0
active = 0
nextevt = T1
/ \
0 1
active idle (T1)
0) CPU 1 is idle and has a timer queued (T1), CPU 0 is active and is
the active migrator.
[GRP0:0]
migrator = NONE
active = NONE
nextevt = T1
/ \
0 1
idle idle (T1)
wakeup = T1
1) CPU 0 is now idle and is therefore the idle migrator. It has
programmed its next timer interrupt to handle T1.
[GRP0:0]
migrator = 1
active = 1
nextevt = KTIME_MAX
/ \
0 1
idle active
wakeup = T1
2) CPU 1 has woken up, it is now active and it has just handled its own
timer T1.
3) CPU 0 gets a timer interrupt to handle T1 but tmigr_handle_remote()
realize it is not the migrator anymore. So it early returns without
observing that T1 has been expired already and therefore without
updating its ->wakeup value.
4) CPU 0 goes into tmigr_cpu_new_timer() which also early returns
because it doesn't queue a timer of its own. So ->wakeup is left
unchanged and the next timer is programmed to fire now.
5) goto 3) forever
This results in timer interrupt storms in idle and also in nohz_full (as
observed in rcutorture's TREE07 scenario).
Fix this with forcing a re-evaluation of tmc->wakeup while trying
remote timer handling when the CPU isn't the migrator anymmore. The
check is inherently racy but in the worst case the CPU just races setting
the KTIME_MAX value that a remote expiry also tries to set.
Fixes:
|
||
|
Frederic Weisbecker
|
4b6f4c5a67 |
timer/migration: Remove buggy early return on deactivation
When a CPU enters into idle and deactivates itself from the timer
migration hierarchy without any global timer of its own to propagate,
the group event of that CPU is set to "ignore" and tmigr_update_events()
accordingly performs an early return without considering timers queued
by other CPUs.
If the hierarchy has a single level, and the CPU is the last one to
enter idle, it will ignore others' global timers, as in the following
layout:
[GRP0:0]
migrator = 0
active = 0
nextevt = T0i
/ \
0 1
active (T0i) idle (T1)
0) CPU 0 is active thus its event is ignored (the letter 'i') and so are
upper levels' events. CPU 1 is idle and has the timer T1 enqueued.
[GRP0:0]
migrator = NONE
active = NONE
nextevt = T0i
/ \
0 1
idle (T0i) idle (T1)
1) CPU 0 goes idle without global event queued. Therefore KTIME_MAX is
pushed as its next expiry and its own event kept as "ignore". As a result
tmigr_update_events() ignores T1 and CPU 0 goes to idle with T1
unhandled.
This isn't proper to single level hierarchy though. A similar issue,
although slightly different, may arise on multi-level:
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0i, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = 0 migrator = NONE
active = 0 active = NONE
nextevt = T0i nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
active idle idle idle
0) CPU 0 is active thus its event is ignored (the letter 'i') and so are
upper levels' events. CPU 1 is idle and has the timer T1 enqueued.
CPU 2 also has a timer. The expiry order is T0 (ignored) < T1 < T2
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0i, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T0i nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
idle idle idle idle
1) CPU 0 goes idle without global event queued. Therefore KTIME_MAX is
pushed as its next expiry and its own event kept as "ignore". As a result
tmigr_update_events() ignores T1. The change only propagated up to 1st
level so far.
[GRP1:0]
migrator = NONE
active = NONE
nextevt = T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T0i nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
idle idle idle idle
2) The change now propagates up to the top. tmigr_update_events() finds
that the child event is ignored and thus removes it. The top level next
event is now T2 which is returned to CPU 0 as its next effective expiry
to take account for as the global idle migrator. However T1 has been
ignored along the way, leaving it unhandled.
Fix those issues with removing the buggy related early return. Ignored
child events must not prevent from evaluating the other events within
the same group.
Reported-by: Boqun Feng <boqun.feng@gmail.com>
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/ZfOhB9ZByTZcBy4u@lothringen
|
||
|
Frederic Weisbecker
|
8ca1836769 |
timer/migration: Fix quick check reporting late expiry
When a CPU is the last active in the hierarchy and it tries to enter
into idle, the quick check looking up the next event towards cpuidle
heuristics may report a too late expiry, such as in the following
scenario:
[GRP1:0]
migrator = NONE
active = NONE
nextevt = T0:0, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T0, T1 nextevt = T2
/ \ / \
0 1 2 3
idle idle idle idle
0) The whole system is idle, and CPU 0 was the last migrator. CPU 0 has
a timer (T0), CPU 1 has a timer (T1) and CPU 2 has a timer (T2). The
expire order is T0 < T1 < T2.
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0(i), T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = CPU0 migrator = NONE
active = CPU0 active = NONE
nextevt = T0(i), T1 nextevt = T2
/ \ / \
0 1 2 3
active idle idle idle
1) CPU 0 becomes active. The (i) means a now ignored timer.
[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = CPU0 migrator = NONE
active = CPU0 active = NONE
nextevt = T1 nextevt = T2
/ \ / \
0 1 2 3
active idle idle idle
2) CPU 0 handles remote. No timer actually expired but ignored timers
have been cleaned out and their sibling's timers haven't been
propagated. As a result the top level's next event is T2 and not T1.
3) CPU 0 tries to enter idle without any global timer enqueued and calls
tmigr_quick_check(). The expiry of T2 is returned instead of the
expiry of T1.
When the quick check returns an expiry that is too late, the cpuidle
governor may pick up a C-state that is too deep. This may be result into
undesired CPU wake up latency if the next timer is actually close enough.
Fix this with assuming that expiries aren't sorted top-down while
performing the quick check. Pick up instead the earliest encountered one
while walking up the hierarchy.
|
||
|
Anna-Maria Behnsen
|
36e40df35d |
timer_migration: Add tracepoints
The timer pull logic needs proper debugging aids. Add tracepoints so the hierarchical idle machinery can be diagnosed. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20240222103403.31923-1-anna-maria@linutronix.de |
||
|
Anna-Maria Behnsen
|
7ee9887703 |
timers: Implement the hierarchical pull model
Placing timers at enqueue time on a target CPU based on dubious heuristics
does not make any sense:
1) Most timer wheel timers are canceled or rearmed before they expire.
2) The heuristics to predict which CPU will be busy when the timer expires
are wrong by definition.
So placing the timers at enqueue wastes precious cycles.
The proper solution to this problem is to always queue the timers on the
local CPU and allow the non pinned timers to be pulled onto a busy CPU at
expiry time.
Therefore split the timer storage into local pinned and global timers:
Local pinned timers are always expired on the CPU on which they have been
queued. Global timers can be expired on any CPU.
As long as a CPU is busy it expires both local and global timers. When a
CPU goes idle it arms for the first expiring local timer. If the first
expiring pinned (local) timer is before the first expiring movable timer,
then no action is required because the CPU will wake up before the first
movable timer expires. If the first expiring movable timer is before the
first expiring pinned (local) timer, then this timer is queued into an idle
timerqueue and eventually expired by another active CPU.
To avoid global locking the timerqueues are implemented as a hierarchy. The
lowest level of the hierarchy holds the CPUs. The CPUs are associated to
groups of 8, which are separated per node. If more than one CPU group
exist, then a second level in the hierarchy collects the groups. Depending
on the size of the system more than 2 levels are required. Each group has a
"migrator" which checks the timerqueue during the tick for remote expirable
timers.
If the last CPU in a group goes idle it reports the first expiring event in
the group up to the next group(s) in the hierarchy. If the last CPU goes
idle it arms its timer for the first system wide expiring timer to ensure
that no timer event is missed.
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240222103710.32582-1-anna-maria@linutronix.de
|