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SubjectRe: perf: add cond_resched() to task_function_call()
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. 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?

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