From: Barret Rhoden <brho@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
linux-kernel@vger.kernel.org,
syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
Subject: Re: perf: add cond_resched() to task_function_call()
Date: Tue, 14 Apr 2020 17:16:05 -0400 [thread overview]
Message-ID: <bbec4199-37ef-fda2-b18f-3d485c1d5b70@google.com> (raw)
In-Reply-To: <20200414204208.GI2483@worktop.programming.kicks-ass.net>
On 4/14/20 4:42 PM, Peter Zijlstra wrote:
> On Tue, Apr 14, 2020 at 03:03:51PM -0400, Barret Rhoden wrote:
>> Under rare circumstances, task_function_call() can repeatedly fail and
>> cause a soft lockup.
>>
>> There is a slight race where the process is no longer running on the cpu
>> we targeted by the time remote_function() runs. The code will simply
>> try again. If we are very unlucky, this will continue to fail, until a
>> watchdog fires. This can happen in a heavily loaded, multi-core virtual
>> machine.
>
> Sigh,.. virt again :/
>
>> Reported-by: syzbot+bb4935a5c09b5ff79940@syzkaller.appspotmail.com
>> Signed-off-by: Barret Rhoden <brho@google.com>
>> ---
>> kernel/events/core.c | 12 ++++++++----
>> 1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 55e44417f66d..65c2c05e24c2 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -99,7 +99,7 @@ static void remote_function(void *data)
>> *
>> * returns: @func return value, or
>> * -ESRCH - when the process isn't running
>> - * -EAGAIN - when the process moved away
>> + * -ENXIO - when the cpu the process was on has gone offline
>> */
>
> Hurm.. I don't think that was actually intended behaviour.
Yeah, I noticed we never return EAGAIN, so I figured I'd look for any
other potential return values.
> As long as
> the task lives we ought to retry. Luckily I don't think the current code
> cares much, it'll loop again on the caller side.
>
> With the exception of perf_cgroup_attach() that is, that might actually
> be broken because of this.
>
>> static int
>> task_function_call(struct task_struct *p, remote_function_f func, void *info)
>> @@ -112,11 +112,15 @@ task_function_call(struct task_struct *p, remote_function_f func, void *info)
>> };
>> int ret;
>>
>> - do {
>> - ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
>> + while (1) {
>> + ret = smp_call_function_single(task_cpu(p), remote_function,
>> + &data, 1);
>> if (!ret)
>> ret = data.ret;
>> - } while (ret == -EAGAIN);
>> + if (ret != -EAGAIN)
>> + break;
>> + cond_resched();
>> + }
>
> So how about we make that:
>
> for (;;) {
> ret = smp_call_function_single(task_cpu(p), remote_function, &data, 1);
> ret = !ret ? data.ret : -EAGAIN;
>
> if (ret != -EAGAIN)
> break;
>
> cond_resched();
> }
>
> Or something like that, hmmm?
Can do. I can put a comment in too. /* Retry due to any failure in
smp_call_function */ or something.
Thanks,
Barret
next prev parent reply other threads:[~2020-04-14 21:16 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-14 19:03 perf: add cond_resched() to task_function_call() Barret Rhoden
2020-04-14 20:42 ` Peter Zijlstra
2020-04-14 21:16 ` Barret Rhoden [this message]
2020-04-14 22:29 ` Barret Rhoden
2020-05-01 18:22 ` [tip: perf/core] perf: Add " tip-bot2 for Barret Rhoden
2020-05-23 9:20 ` Jiri Olsa
2020-05-23 9:27 ` Jiri Olsa
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