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Message-ID: <20170111195100.GA3127@twins.programming.kicks-ass.net>
Date: Wed, 11 Jan 2017 20:51:00 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
jeremy.linton@....com, Will Deacon <will.deacon@....com>
Subject: Re: Perf hotplug lockup in v4.9-rc8
On Wed, Jan 11, 2017 at 05:03:58PM +0100, Peter Zijlstra wrote:
>
> In any case, I'll try and write a proper Changelog for this...
This is what I came up with, most of it should look familiar, its
copy/pasted bits from this thread.
---
Subject: perf: Fix sys_perf_event_open() vs hotplug
From: Peter Zijlstra <peterz@...radead.org>
Date: Fri, 9 Dec 2016 14:59:00 +0100
There is problem with installing an event in a task that is 'stuck' on
an offline CPU.
Blocked tasks are not dis-assosciated from offlined CPUs, after all, a
blocked task doesn't run and doesn't require a CPU etc.. Only on
wakeup do we ammend the situation and place the task on a available
CPU.
If we hit such a task with perf_install_in_context() we'll loop until
either that task wakes up or the CPU comes back online, if the task
waking depends on the event being installed, we're stuck.
While looking into this issue, I also spotted another problem, if we
hit a task with perf_install_in_context() that is in the middle of
being migrated, that is we observe the old CPU before sending the IPI,
but run the IPI (on the old CPU) while the task is already running on
the new CPU, things also go sideways.
Rework things to rely on task_curr() -- outside of rq->lock -- which
is rather tricky. Imagine the following scenario where we're trying to
install the first event into our task 't':
CPU0 CPU1 CPU2
(current == t)
t->perf_event_ctxp[] = ctx;
smp_mb();
cpu = task_cpu(t);
switch(t, n);
migrate(t, 2);
switch(p, t);
ctx = t->perf_event_ctxp[]; // must not be NULL
smp_function_call(cpu, ..);
generic_exec_single()
func();
spin_lock(ctx->lock);
if (task_curr(t)) // false
add_event_to_ctx();
spin_unlock(ctx->lock);
perf_event_context_sched_in();
spin_lock(ctx->lock);
// sees event
So its CPU0's store of t->perf_event_ctxp[] that must not go 'missing'.
Because if CPU2's load of that variable were to observe NULL, it would
not try to schedule the ctx and we'd have a task running without its
counter, which would be 'bad'.
As long as we observe !NULL, we'll acquire ctx->lock. If we acquire it
first and not see the event yet, then CPU0 must observe task_curr()
and retry. If the install happens first, then we must see the event on
sched-in and all is well.
I think we can translate the first part (until the 'must not be NULL')
of the scenario to a litmus test like:
C C-peterz
{
}
P0(int *x, int *y)
{
int r1;
WRITE_ONCE(*x, 1);
smp_mb();
r1 = READ_ONCE(*y);
}
P1(int *y, int *z)
{
WRITE_ONCE(*y, 1);
smp_store_release(z, 1);
}
P2(int *x, int *z)
{
int r1;
int r2;
r1 = smp_load_acquire(z);
smp_mb();
r2 = READ_ONCE(*x);
}
exists
(0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
Where:
x is perf_event_ctxp[],
y is our tasks's CPU, and
z is our task being placed on the rq of CPU2.
The P0 smp_mb() is the one added by this patch, ordering the store to
perf_event_ctxp[] from find_get_context() and the load of task_cpu()
in task_function_call().
The smp_store_release/smp_load_acquire model the RCpc locking of the
rq->lock and the smp_mb() of P2 is the context switch switching from
whatever CPU2 was running to our task 't'.
This litmus test evaluates into:
Test C-peterz Allowed
States 7
0:r1=0; 2:r1=0; 2:r2=0;
0:r1=0; 2:r1=0; 2:r2=1;
0:r1=0; 2:r1=1; 2:r2=1;
0:r1=1; 2:r1=0; 2:r2=0;
0:r1=1; 2:r1=0; 2:r2=1;
0:r1=1; 2:r1=1; 2:r2=0;
0:r1=1; 2:r1=1; 2:r2=1;
No
Witnesses
Positive: 0 Negative: 7
Condition exists (0:r1=0 /\ 2:r1=1 /\ 2:r2=0)
Observation C-peterz Never 0 7
Hash=e427f41d9146b2a5445101d3e2fcaa34
And the strong and weak model agree.
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Will Deacon <will.deacon@....com>
Cc: jeremy.linton@....com
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reported-by: Mark Rutland <mark.rutland@....com>
Tested-by: Mark Rutland <mark.rutland@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20161209135900.GU3174@twins.programming.kicks-ass.net
---
kernel/events/core.c | 70 ++++++++++++++++++++++++++++++++++-----------------
1 file changed, 48 insertions(+), 22 deletions(-)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2249,7 +2249,7 @@ static int __perf_install_in_context(vo
struct perf_event_context *ctx = event->ctx;
struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
struct perf_event_context *task_ctx = cpuctx->task_ctx;
- bool activate = true;
+ bool reprogram = true;
int ret = 0;
raw_spin_lock(&cpuctx->ctx.lock);
@@ -2257,27 +2257,26 @@ static int __perf_install_in_context(vo
raw_spin_lock(&ctx->lock);
task_ctx = ctx;
- /* If we're on the wrong CPU, try again */
- if (task_cpu(ctx->task) != smp_processor_id()) {
- ret = -ESRCH;
- goto unlock;
- }
+ reprogram = (ctx->task == current);
/*
- * If we're on the right CPU, see if the task we target is
- * current, if not we don't have to activate the ctx, a future
- * context switch will do that for us.
+ * If the task is running, it must be running on this CPU,
+ * otherwise we cannot reprogram things.
+ *
+ * If its not running, we don't care, ctx->lock will
+ * serialize against it becoming runnable.
*/
- if (ctx->task != current)
- activate = false;
- else
- WARN_ON_ONCE(cpuctx->task_ctx && cpuctx->task_ctx != ctx);
+ if (task_curr(ctx->task) && !reprogram) {
+ ret = -ESRCH;
+ goto unlock;
+ }
+ WARN_ON_ONCE(reprogram && cpuctx->task_ctx && cpuctx->task_ctx != ctx);
} else if (task_ctx) {
raw_spin_lock(&task_ctx->lock);
}
- if (activate) {
+ if (reprogram) {
ctx_sched_out(ctx, cpuctx, EVENT_TIME);
add_event_to_ctx(event, ctx);
ctx_resched(cpuctx, task_ctx);
@@ -2328,13 +2327,36 @@ perf_install_in_context(struct perf_even
/*
* Installing events is tricky because we cannot rely on ctx->is_active
* to be set in case this is the nr_events 0 -> 1 transition.
+ *
+ * Instead we use task_curr(), which tells us if the task is running.
+ * However, since we use task_curr() outside of rq::lock, we can race
+ * against the actual state. This means the result can be wrong.
+ *
+ * If we get a false positive, we retry, this is harmless.
+ *
+ * If we get a false negative, things are complicated. If we are after
+ * perf_event_context_sched_in() ctx::lock will serialize us, and the
+ * value must be correct. If we're before, it doesn't matter since
+ * perf_event_context_sched_in() will program the counter.
+ *
+ * However, this hinges on the remote context switch having observed
+ * our task->perf_event_ctxp[] store, such that it will in fact take
+ * ctx::lock in perf_event_context_sched_in().
+ *
+ * We do this by task_function_call(), if the IPI fails to hit the task
+ * we know any future context switch of task must see the
+ * perf_event_ctpx[] store.
*/
-again:
+
/*
- * Cannot use task_function_call() because we need to run on the task's
- * CPU regardless of whether its current or not.
+ * This smp_mb() orders the task->perf_event_ctxp[] store with the
+ * task_cpu() load, such that if the IPI then does not find the task
+ * running, a future context switch of that task must observe the
+ * store.
*/
- if (!cpu_function_call(task_cpu(task), __perf_install_in_context, event))
+ smp_mb();
+again:
+ if (!task_function_call(task, __perf_install_in_context, event))
return;
raw_spin_lock_irq(&ctx->lock);
@@ -2348,12 +2370,16 @@ perf_install_in_context(struct perf_even
raw_spin_unlock_irq(&ctx->lock);
return;
}
- raw_spin_unlock_irq(&ctx->lock);
/*
- * Since !ctx->is_active doesn't mean anything, we must IPI
- * unconditionally.
+ * If the task is not running, ctx->lock will avoid it becoming so,
+ * thus we can safely install the event.
*/
- goto again;
+ if (task_curr(task)) {
+ raw_spin_unlock_irq(&ctx->lock);
+ goto again;
+ }
+ add_event_to_ctx(event, ctx);
+ raw_spin_unlock_irq(&ctx->lock);
}
/*
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