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

[catalin.marinas@arm.com (Catalin Marinas)]

On Tue, Apr 19, 2016 at 06:23:20PM +0100, Will Deacon wrote:
> 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 <lorenzo.pieralisi@arm.com>
> > Reported-by: James Morse <james.morse@arm.com>
> > Cc: Ashwin Chaugule <ashwin.chaugule@linaro.org>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Kevin Hilman <khilman@baylibre.com>
> > Cc: Sudeep Holla <sudeep.holla@arm.com>
> > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> > Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > ---
> >  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 <will.deacon@arm.com>
> 
> I assume Catalin will take this as a fix?

Applied. Thanks.

-- 
Catalin