From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Feng Tang <feng.tang@intel.com>, Oleg Nesterov <oleg@redhat.com>,
Jiri Olsa <jolsa@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
kernel test robot <rong.a.chen@intel.com>,
Ingo Molnar <mingo@kernel.org>,
Vince Weaver <vincent.weaver@maine.edu>,
Jiri Olsa <jolsa@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Arnaldo Carvalho de Melo <acme@redhat.com>,
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
Stephane Eranian <eranian@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
lkp@lists.01.org, andi.kleen@intel.com, "Huang,
Ying" <ying.huang@intel.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression
Date: Mon, 24 Feb 2020 13:43:02 -0800 [thread overview]
Message-ID: <CAHk-=wh0z0LErNzwe-AqrEkv3BNzJep58Qmi2dM775UPtmq0og@mail.gmail.com> (raw)
In-Reply-To: <87a757znqd.fsf@x220.int.ebiederm.org>
On Mon, Feb 24, 2020 at 1:22 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> I keep looking at your patch and wondering if there isn't a way
> to remove the uid refcount entirely on this path.
I agree. I tried to come up with something, but couldn't.
> Linus I might be wrong but I have this sense that your change will only
> help when signal delivery is backed up. I expect in the common case
> there won't be any pending signals outstanding for the user.
Again, 100% agreed.
HOWEVER.
The normal case where there's one only signal pending is also the case
where we don't care about the extra atomic RMW access. By definition
that's not going to ever going to show up as a performance issue or
for cacheline contention.
So the only case that matters from a performance standpoint is the
"lots of signals" case, in which case you'll see that sigqueue become
backed up.
But as I said in the original thread (before you got added to the list):
"I don't know. This does not seem to be a particularly serious load."
I'm not convinced this will show up outside of this kind of
signal-sending microbenchmark.
That said, I don't really see any downside to the patch either, so...
Linus
next prev parent reply other threads:[~2020-02-24 21:50 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-05 12:32 [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression kernel test robot
2020-02-05 12:58 ` Peter Zijlstra
2020-02-06 3:04 ` [LKP] " Li, Philip
2020-02-21 8:03 ` Feng Tang
2020-02-21 10:58 ` Peter Zijlstra
2020-02-21 13:20 ` Jiri Olsa
2020-02-23 14:11 ` Feng Tang
2020-02-23 17:37 ` Linus Torvalds
2020-02-24 0:33 ` Feng Tang
2020-02-24 1:06 ` Linus Torvalds
2020-02-24 1:58 ` Huang, Ying
2020-02-24 2:19 ` Feng Tang
2020-02-24 13:20 ` Feng Tang
2020-02-24 19:24 ` Linus Torvalds
2020-02-24 19:42 ` Kleen, Andi
2020-02-24 20:09 ` Linus Torvalds
2020-02-24 20:47 ` Linus Torvalds
2020-02-24 21:20 ` Eric W. Biederman
2020-02-24 21:43 ` Linus Torvalds [this message]
2020-02-24 21:59 ` Eric W. Biederman
2020-02-24 22:12 ` Linus Torvalds
2020-02-25 2:57 ` Feng Tang
2020-02-25 3:15 ` Linus Torvalds
2020-02-25 4:53 ` Feng Tang
2020-02-23 19:36 ` Jiri Olsa
2020-02-21 18:05 ` Kleen, Andi
2020-02-22 12:43 ` Feng Tang
2020-02-22 17:08 ` Kleen, Andi
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