Re: [PATCH] timer/migration: Remove buggy early return on deactivation [was Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU changes for v6.9]]

Florian Fainelli
2024-03-15T13:45:00Z

On 3/14/2024 6:14 PM, Frederic Weisbecker wrote:
> On Thu, Mar 14, 2024 at 03:05:53PM -0700, Boqun Feng wrote:
>> I notice CPU3 didn't have its own non-deferrable timer queued (local or
>> global), so could the following happen?
>>
>> timer_base_try_to_set_idle():
>> __get_next_timer_interrupt():
>> fetch_next_timer_interrupt():
>> // nextevt_local == nextevt_global == basej + NEXT_TIMER_MAX_DELTA
>> // tevt->local == tevt->gloabl = KTIME_MAX
>> timer_use_tmigr():
>> tmigr_cpu_deactivate():
>> __tmigr_cpu_deactivate():
>> // tmc->cpuevt.ignore untouched still == true
>> walk_groups(&tmigr_inactive_up, ...):
>> tmigr_inactive_up():
>> data->remote = true;
>> tmigr_update_events():
>> if (child) { // child is NULL
>> ...
>> } else {
>> first_childevt = evt = data->evt;
>>
>> if (evt->ignore && !remote)
>> return true; // no remote tick is picked.
>> ...
>> }
>
> Nice catch! Florian can you try the following?
>
> From b0e335371ed758f68bf4f501246298c98a615b04 Mon Sep 17 00:00:00 2001
> From: Frederic Weisbecker <frederic@kernel.org>
> Date: Fri, 15 Mar 2024 00:21:01 +0100
> Subject: [PATCH] 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
> idle 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>

Gave it a good 20 cold boots and suspend to DRAM cycles and none of
those triggered the reported behavior:

Reported-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>

Thanks for the quick fix Frederic!
--
Florian