Messages in this thread | | | Date | Tue, 14 Apr 2020 22:42:08 +0200 | From | Peter Zijlstra <> | Subject | Re: perf: add cond_resched() to task_function_call() |
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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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