Re: [PATCH 2/2] getrusage: use sig->stats_lock

[Dylan Hatch]

On Fri, Jan 19, 2024 at 6:16 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> Rather than lock_task_sighand(), sig->stats_lock was specifically designed
> for this type of use. This way getrusage runs lockless in the likely case.
>
> TODO:
>         - Change do_task_stat() to use sig->stats_lock too, then we can
>           remove spin_lock_irq(siglock) in wait_task_zombie().
>
>         - Turn sig->stats_lock into seqcount_rwlock_t, this way the
>           readers in the slow mode won't exclude each other. See
>           https://lore.kernel.org/all/20230913154907.GA26210@xxxxxxxxxx/
>
>         - stats_lock has to disable irqs because ->siglock can be taken
>           in irq context, it would be very nice to change __exit_signal()
>           to avoid the siglock->stats_lock dependency.
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
>  kernel/sys.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 70ad06ad852e..f8e543f1e38a 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -1788,7 +1788,9 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>         unsigned long maxrss;
>         struct mm_struct *mm;
>         struct signal_struct *sig = p->signal;
> +       unsigned int seq = 0;
>
> +retry:
>         memset(r, 0, sizeof(*r));
>         utime = stime = 0;
>         maxrss = 0;
> @@ -1800,8 +1802,7 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>                 goto out_thread;
>         }
>
> -       if (!lock_task_sighand(p, &flags))
> -               return;
> +       flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
>
>         switch (who) {
>         case RUSAGE_BOTH:
> @@ -1829,14 +1830,23 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
>                 r->ru_oublock += sig->oublock;
>                 if (maxrss < sig->maxrss)
>                         maxrss = sig->maxrss;
> +
> +               rcu_read_lock();
>                 __for_each_thread(sig, t)
>                         accumulate_thread_rusage(t, r);
> +               rcu_read_unlock();
> +
>                 break;
>
>         default:
>                 BUG();
>         }
> -       unlock_task_sighand(p, &flags);
> +
> +       if (need_seqretry(&sig->stats_lock, seq)) {
> +               seq = 1;
> +               goto retry;
> +       }
> +       done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
>
>         if (who == RUSAGE_CHILDREN)
>                 goto out_children;
> --
> 2.25.1.362.g51ebf55
>
>

I applied these to a 5.10 kernel, and my repro (calling getrusage(RUSAGE_SELF)
from 200K threads) is no longer triggering a hard lockup.

Tested-by: Dylan Hatch <dylanbhatch@xxxxxxxxxx>