From: Tim Chen <tim.c.chen@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@elte.hu>, Kan Liang <kan.liang@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>,
linux-mm <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk
Date: Tue, 15 Aug 2017 12:05:40 -0700 [thread overview]
Message-ID: <0b7b6132-a374-9636-53f9-c2e1dcec230f@linux.intel.com> (raw)
In-Reply-To: <CA+55aFw1A1C8qUeKPUzACrsqn97UDxTP3M2SRs80aEztfU=Qbg@mail.gmail.com>
On 08/14/2017 08:28 PM, Linus Torvalds wrote:
> On Mon, Aug 14, 2017 at 8:15 PM, Andi Kleen <ak@linux.intel.com> wrote:
>> But what should we do when some other (non page) wait queue runs into the
>> same problem?
>
> Hopefully the same: root-cause it.
>
> Once you have a test-case, it should generally be fairly simple to do
> with profiles, just seeing who the caller is when ttwu() (or whatever
> it is that ends up being the most noticeable part of the wakeup chain)
> shows up very heavily.
We have a test case but it is a customer workload. We'll try to get
a bit more info.
>
> And I think that ends up being true whether the "break up long chains"
> patch goes in or not. Even if we end up allowing interrupts in the
> middle, a long wait-queue is a problem.
>
> I think the "break up long chains" thing may be the right thing
> against actual malicious attacks, but not for any actual real
> benchmark or load.
This is a concern from our customer as we could trigger the watchdog timer
by running user space workloads.
>
> I don't think we normally have cases of long wait-queues, though. At
> least not the kinds that cause problems. The real (and valid)
> thundering herd cases should already be using exclusive waiters that
> only wake up one process at a time.
>
> The page bit-waiting is hopefully special. As mentioned, we used to
> have some _really_ special code for it for other reasons, and I
> suspect you see this problem with them because we over-simplified it
> from being a per-zone dynamically sized one (where the per-zone thing
> caused both performance problems and actual bugs) to being that
> "static small array".
>
> So I think/hope that just re-introducing some dynamic sizing will help
> sufficiently, and that this really is an odd and unusual case.
I agree that dynamic sizing makes a lot of sense. We'll check to
see if additional size to the hash table helps, assuming that the
waiters are distributed among different pages for our test case.
Thanks.
Tim
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next prev parent reply other threads:[~2017-08-15 19:05 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-15 0:52 [PATCH 1/2] sched/wait: Break up long wake list walk Tim Chen
2017-08-15 0:52 ` [PATCH 2/2] sched/wait: Introduce lock breaker in wake_up_page_bit Tim Chen
2017-08-15 1:48 ` [PATCH 1/2] sched/wait: Break up long wake list walk Linus Torvalds
2017-08-15 2:27 ` Andi Kleen
2017-08-15 2:52 ` Linus Torvalds
2017-08-15 3:15 ` Andi Kleen
2017-08-15 3:28 ` Linus Torvalds
2017-08-15 19:05 ` Tim Chen [this message]
2017-08-15 19:41 ` Linus Torvalds
2017-08-15 19:47 ` Linus Torvalds
2017-08-15 22:47 ` Davidlohr Bueso
2017-08-15 22:56 ` Linus Torvalds
2017-08-15 22:57 ` Linus Torvalds
2017-08-15 23:50 ` Linus Torvalds
2017-08-16 23:22 ` Eric W. Biederman
2017-08-17 16:17 ` Liang, Kan
2017-08-17 16:25 ` Linus Torvalds
2017-08-17 20:18 ` Liang, Kan
2017-08-17 20:44 ` Linus Torvalds
2017-08-18 12:23 ` Mel Gorman
2017-08-18 14:20 ` Liang, Kan
2017-08-18 14:46 ` Mel Gorman
2017-08-18 16:36 ` Tim Chen
2017-08-18 16:45 ` Andi Kleen
2017-08-18 16:53 ` Liang, Kan
2017-08-18 17:48 ` Linus Torvalds
2017-08-18 18:54 ` Mel Gorman
2017-08-18 19:14 ` Linus Torvalds
2017-08-18 19:58 ` Andi Kleen
2017-08-18 20:10 ` Linus Torvalds
2017-08-21 18:32 ` Mel Gorman
2017-08-21 18:56 ` Liang, Kan
2017-08-22 17:23 ` Liang, Kan
2017-08-22 18:19 ` Linus Torvalds
2017-08-22 18:25 ` Linus Torvalds
2017-08-22 18:56 ` Peter Zijlstra
2017-08-22 19:15 ` Linus Torvalds
2017-08-22 19:08 ` Peter Zijlstra
2017-08-22 19:30 ` Linus Torvalds
2017-08-22 19:37 ` Andi Kleen
2017-08-22 21:08 ` Christopher Lameter
2017-08-22 21:24 ` Andi Kleen
2017-08-22 22:52 ` Linus Torvalds
2017-08-22 23:19 ` Linus Torvalds
2017-08-23 14:51 ` Liang, Kan
2017-08-22 19:55 ` Liang, Kan
2017-08-22 20:42 ` Linus Torvalds
2017-08-22 20:53 ` Peter Zijlstra
2017-08-22 20:58 ` Linus Torvalds
2017-08-23 14:49 ` Liang, Kan
2017-08-23 15:58 ` Tim Chen
2017-08-23 18:17 ` Linus Torvalds
2017-08-23 20:55 ` Liang, Kan
2017-08-23 23:30 ` Linus Torvalds
2017-08-24 17:49 ` Tim Chen
2017-08-24 18:16 ` Linus Torvalds
2017-08-24 20:44 ` Mel Gorman
2017-08-25 16:44 ` Tim Chen
2017-08-23 16:04 ` Mel Gorman
2017-08-18 20:05 ` Andi Kleen
2017-08-18 20:29 ` Linus Torvalds
2017-08-18 20:29 ` Liang, Kan
2017-08-18 20:34 ` Linus Torvalds
2017-08-18 16:55 ` Linus Torvalds
2017-08-18 13:06 ` Liang, Kan
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