On Mon, Dec 16, 2013 at 04:26:36PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 16, 2013 at 07:19:22AM -0800, tip-bot for Paul E. McKenney wrote:
> > The underlying problem is that perf is invoking call_rcu() with the
> > scheduler locks held, but in NOCB mode, call_rcu() will with high
> > probability invoke the scheduler -- which just might want to use its
> > locks. The reason that call_rcu() needs to invoke the scheduler is
> > to wake up the corresponding rcuo callback-offload kthread, which
> > does the job of starting up a grace period and invoking the callbacks
> > afterwards.
> >
> > One solution (championed on a related problem by Lai Jiangshan) is to
> > simply defer the wakeup to some point where scheduler locks are no longer
> > held. Since we don't want to unnecessarily incur the cost of such
> > deferral, the task before us is threefold:
> >
> > 1. Determine when it is likely that a relevant scheduler lock is held.
> >
> > 2. Defer the wakeup in such cases.
> >
> > 3. Ensure that all deferred wakeups eventually happen, preferably
> > sooner rather than later.
> >
> > We use irqs_disabled_flags() as a proxy for relevant scheduler locks
> > being held. This works because the relevant locks are always acquired
> > with interrupts disabled. We may defer more often than needed, but that
> > is at least safe.
>
> This would also allow us to do away with things like the below patch,
> right?
It takes care of one problem, but there are others, including
rcu_read_unlock() inovking the scheduler to deboost itself. So for the
moment, we still need the below patch.
Thanx, Paul
> ---
> commit 058ebd0eba3aff16b144eabf4510ed9510e1416e
> Author: Peter Zijlstra <[email protected]>
> Date: Fri Jul 12 11:08:33 2013 +0200
>
> perf: Fix perf_lock_task_context() vs RCU
>
> Jiri managed to trigger this warning:
>
> [] ======================================================
> [] [ INFO: possible circular locking dependency detected ]
> [] 3.10.0+ #228 Tainted: G W
> [] -------------------------------------------------------
> [] p/6613 is trying to acquire lock:
> [] (rcu_node_0){..-...}, at: [<ffffffff810ca797>]
> rcu_read_unlock_special+0xa7/0x250
> []
> [] but task is already holding lock:
> [] (&ctx->lock){-.-...}, at: [<ffffffff810f2879>]
> perf_lock_task_context+0xd9/0x2c0
> []
> [] which lock already depends on the new lock.
> []
> [] the existing dependency chain (in reverse order) is:
> []
> [] -> #4 (&ctx->lock){-.-...}:
> [] -> #3 (&rq->lock){-.-.-.}:
> [] -> #2 (&p->pi_lock){-.-.-.}:
> [] -> #1 (&rnp->nocb_gp_wq[1]){......}:
> [] -> #0 (rcu_node_0){..-...}:
>
> Paul was quick to explain that due to preemptible RCU we cannot call
> rcu_read_unlock() while holding scheduler (or nested) locks when part
> of the read side critical section was preemptible.
>
> Therefore solve it by making the entire RCU read side non-preemptible.
>
> Also pull out the retry from under the non-preempt to play nice with RT.
>
> Reported-by: Jiri Olsa <[email protected]>
> Helped-out-by: Paul E. McKenney <[email protected]>
> Cc: <[email protected]>
> Signed-off-by: Peter Zijlstra <[email protected]>
> Signed-off-by: Ingo Molnar <[email protected]>
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index ef5e7cc686e3..eba8fb5834ae 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -947,8 +947,18 @@ perf_lock_task_context(struct task_struct *task, int
> ctxn, unsigned long *flags)
> {
> struct perf_event_context *ctx;
>
> - rcu_read_lock();
> retry:
> + /*
> + * One of the few rules of preemptible RCU is that one cannot do
> + * rcu_read_unlock() while holding a scheduler (or nested) lock when
> + * part of the read side critical section was preemptible -- see
> + * rcu_read_unlock_special().
> + *
> + * Since ctx->lock nests under rq->lock we must ensure the entire read
> + * side critical section is non-preemptible.
> + */
> + preempt_disable();
> + rcu_read_lock();
> ctx = rcu_dereference(task->perf_event_ctxp[ctxn]);
> if (ctx) {
> /*
> @@ -964,6 +974,8 @@ perf_lock_task_context(struct task_struct *task, int
> ctxn, unsigned long *flags)
> raw_spin_lock_irqsave(&ctx->lock, *flags);
> if (ctx != rcu_dereference(task->perf_event_ctxp[ctxn])) {
> raw_spin_unlock_irqrestore(&ctx->lock, *flags);
> + rcu_read_unlock();
> + preempt_enable();
> goto retry;
> }
>
> @@ -973,6 +985,7 @@ perf_lock_task_context(struct task_struct *task, int
> ctxn, unsigned long *flags)
> }
> }
> rcu_read_unlock();
> + preempt_enable();
> return ctx;
> }
>
>
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