Messages in this thread | | | Date | Tue, 25 Feb 2020 10:57:48 +0800 | From | Feng Tang <> | Subject | Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression |
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On Mon, Feb 24, 2020 at 12:47:14PM -0800, Linus Torvalds wrote: > [ Adding a few more people that tend to be involved in signal > handling. Just in case - even if they probably don't care ] > > On Mon, Feb 24, 2020 at 12:09 PM Linus Torvalds > > I've tested it, and the profiles on the silly microbenchmark look much > nicer. Now it's just the sigpending update shows up, the refcount case > clearly still occasionally happens, but it's now in the noise. > > I made slight changes to the __sigqueue_alloc() case to generate > better code: since we now use that atomic_inc_return() anyway, we > might as well then use the value that is returned for the > RLIMIT_SIGPENDING check too, instead of reading it again. > > That might avoid another potential cacheline bounce, plus the > generated code just looks better. > > Updated (and now slightly tested!) patch attached. > It would be interesting if this is noticeable on your benchmark > numbers. I didn't actually _time_ anything, I just looked at profiles. > > But my setup clearly isn't going to see the horrible contention case > anyway, so my timing numbers wouldn't be all that interesting.
Thanks for the optimization patch for signal!
It makes a big difference, that the performance score is tripled! bump from original 17000 to 54000. Also the gap between 5.0-rc6 and 5.0-rc6+Jiri's patch is reduced to around 2%.
The test I run is inserting your patch right before 5.0-rc6, then run the test for 5.0-rc6 and 5.0-rc6+Jiri's patch. Sorry it took quite some time, as the test platform is not local but inside 0day's framework, which takes some time for scheduling, kbuilding and testing.
Thanks, Feng
> Linus
> kernel/signal.c | 23 ++++++++++++++--------- > 1 file changed, 14 insertions(+), 9 deletions(-) > > diff --git a/kernel/signal.c b/kernel/signal.c > index 9ad8dea93dbb..5b2396350dd1 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -413,27 +413,32 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi > { > struct sigqueue *q = NULL; > struct user_struct *user; > + int sigpending; > > /* > * Protect access to @t credentials. This can go away when all > * callers hold rcu read lock. > + * > + * NOTE! A pending signal will hold on to the user refcount, > + * and we get/put the refcount only when the sigpending count > + * changes from/to zero. > */ > rcu_read_lock(); > - user = get_uid(__task_cred(t)->user); > - atomic_inc(&user->sigpending); > + user = __task_cred(t)->user; > + sigpending = atomic_inc_return(&user->sigpending); > + if (sigpending == 1) > + get_uid(user); > rcu_read_unlock(); > > - if (override_rlimit || > - atomic_read(&user->sigpending) <= > - task_rlimit(t, RLIMIT_SIGPENDING)) { > + if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) { > q = kmem_cache_alloc(sigqueue_cachep, flags); > } else { > print_dropped_signal(sig); > } > > if (unlikely(q == NULL)) { > - atomic_dec(&user->sigpending); > - free_uid(user); > + if (atomic_dec_and_test(&user->sigpending)) > + free_uid(user); > } else { > INIT_LIST_HEAD(&q->list); > q->flags = 0; > @@ -447,8 +452,8 @@ static void __sigqueue_free(struct sigqueue *q) > { > if (q->flags & SIGQUEUE_PREALLOC) > return; > - atomic_dec(&q->user->sigpending); > - free_uid(q->user); > + if (atomic_dec_and_test(&q->user->sigpending)) > + free_uid(q->user); > kmem_cache_free(sigqueue_cachep, q); > } >
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