Subject: [PATCH] drivers/perf: arm-pmu: fix RCU usage on resume from idle states

From: [email-masked] (Will Deacon)
Date: Tue, 19 Apr 2016 18:23:20 +0100

Hi Lorenzo,

On Tue, Apr 19, 2016 at 06:08:09PM +0100, Lorenzo Pieralisi wrote:
> Commit da4e4f18afe0 ("drivers/perf: arm_pmu: implement CPU_PM notifier")
> added code in the arm perf infrastructure that allows the kernel to
> save/restore perf counters whenever the CPU enters a low-power idle
> state. The kernel saves/restores the counters for each active event
> through the armpmu_{stop/start} ARM pmu API, so that the idle state
> enter/exit power cycle is emulated through pmu start/stop operations
> for each event in use.
> 
> However, calling armpmu_start() for each active event on power up
> executes code that requires RCU locking (perf_event_update_userpage())
> to be functional, so, given that the core may call the CPU_PM notifiers
> while running the idle thread in an quiescent RCU state this is not
> allowed as detected through the following splat when kernel is run with
> CONFIG_PROVE_LOCKING enabled:
> 
> [   49.293286]
> [   49.294761] ===============================
> [   49.298895] [ INFO: suspicious RCU usage. ]
> [   49.303031] 4.6.0-rc3+ #421 Not tainted
> [   49.306821] -------------------------------
> [   49.310956] include/linux/rcupdate.h:872 rcu_read_lock() used
> illegally while idle!
> [   49.318530]
> [   49.318530] other info that might help us debug this:
> [   49.318530]
> [   49.326451]
> [   49.326451] RCU used illegally from idle CPU!
> [   49.326451] rcu_scheduler_active = 1, debug_locks = 0
> [   49.337209] RCU used illegally from extended quiescent state!
> [   49.342892] 2 locks held by swapper/2/0:
> [   49.346768]  #0:  (cpu_pm_notifier_lock){......}, at:
> [<ffffff8008163c28>] cpu_pm_exit+0x18/0x80
> [   49.355492]  #1:  (rcu_read_lock){......}, at: [<ffffff800816dc38>]
> perf_event_update_userpage+0x0/0x260
> 
> This patch refactors the perf CPU_PM notifiers to add a boolean
> flag to the function updating the counters event period, so that the
> userpage update can be skipped when resuming from low-power whilst
> keeping correct save/restore functionality for the running events.
> 
> As a side effect the kernel, while resuming from low-power with
> perf events enabled, runs with a userspace view of active counters that
> is not up-to-date with the kernel one, but since the core power down is
> not really a PMU event start/stop this can be considered acceptable and
> the userspace event snapshot will update the user view of counters
> on subsequent perf event updates requested by either the perf API
> or event counters overflow-triggered interrupts.
> 
> Fixes: da4e4f18afe0 ("drivers/perf: arm_pmu: implement CPU_PM notifier")
> Signed-off-by: Lorenzo Pieralisi <[email-masked]>
> Reported-by: James Morse <[email-masked]>
> Cc: Ashwin Chaugule <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Kevin Hilman <[email-masked]>
> Cc: Sudeep Holla <[email-masked]>
> Cc: Daniel Lezcano <[email-masked]>
> Cc: Mathieu Poirier <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> ---
>  drivers/perf/arm_pmu.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)

This is horrible, but I think it's the best we can do without completely
redesigning the way in which we save/restore the PMU state. We should do
that, but not for 4.6!

Acked-by: Will Deacon <[email-masked]>

I assume Catalin will take this as a fix?

Will