From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f70.google.com (mail-oi0-f70.google.com [209.85.218.70]) by kanga.kvack.org (Postfix) with ESMTP id 9E0A8280704 for ; Tue, 22 Aug 2017 14:19:19 -0400 (EDT) Received: by mail-oi0-f70.google.com with SMTP id a79so8575895oii.7 for ; Tue, 22 Aug 2017 11:19:19 -0700 (PDT) Received: from mail-oi0-x236.google.com (mail-oi0-x236.google.com. [2607:f8b0:4003:c06::236]) by mx.google.com with ESMTPS id l144si12070574oig.201.2017.08.22.11.19.13 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Aug 2017 11:19:13 -0700 (PDT) Received: by mail-oi0-x236.google.com with SMTP id j144so66266741oib.1 for ; Tue, 22 Aug 2017 11:19:13 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <37D7C6CF3E00A74B8858931C1DB2F0775378A24A@SHSMSX103.ccr.corp.intel.com> References: <37D7C6CF3E00A74B8858931C1DB2F0775378761B@SHSMSX103.ccr.corp.intel.com> <20170818122339.24grcbzyhnzmr4qw@techsingularity.net> <37D7C6CF3E00A74B8858931C1DB2F077537879BB@SHSMSX103.ccr.corp.intel.com> <20170818144622.oabozle26hasg5yo@techsingularity.net> <37D7C6CF3E00A74B8858931C1DB2F07753787AE4@SHSMSX103.ccr.corp.intel.com> <20170818185455.qol3st2nynfa47yc@techsingularity.net> <20170821183234.kzennaaw2zt2rbwz@techsingularity.net> <37D7C6CF3E00A74B8858931C1DB2F07753788B58@SHSMSX103.ccr.corp.intel.com> <37D7C6CF3E00A74B8858931C1DB2F0775378A24A@SHSMSX103.ccr.corp.intel.com> From: Linus Torvalds Date: Tue, 22 Aug 2017 11:19:12 -0700 Message-ID: Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk Content-Type: multipart/mixed; boundary="001a113d0a8ae0f4c305575ba1bf" Sender: owner-linux-mm@kvack.org List-ID: To: "Liang, Kan" Cc: Mel Gorman , Mel Gorman , "Kirill A. Shutemov" , Tim Chen , Peter Zijlstra , Ingo Molnar , Andi Kleen , Andrew Morton , Johannes Weiner , Jan Kara , linux-mm , Linux Kernel Mailing List --001a113d0a8ae0f4c305575ba1bf Content-Type: text/plain; charset="UTF-8" On Tue, Aug 22, 2017 at 10:23 AM, Liang, Kan wrote: > > Although the patch doesn't trigger watchdog, the spin lock wait time > is not small (0.45s). > It may get worse again on larger systems. Yeah, I don't think Mel's patch is great - because I think we could do so much better. What I like about Mel's patch is that it recognizes that "wait_on_page_locked()" there is special, and replaces it with something else. I think that "something else" is worse than my "yield()" call, though. In particular, it wastes CPU time even in the good case, and the process that will unlock the page may actually be waiting for us to reschedule. It may be CPU bound, but it might well have just been preempted out. So if we do busy loops, I really think we should also make sure that the thing we're waiting for is not preempted. HOWEVER, I'm actually starting to think that there is perhaps something else going on. Let me walk you through my thinking: This is the migration logic: (a) migration locks the page (b) migration is supposedly CPU-limited (c) migration then unlocks the page. Ignore all the details, that's the 10.000 ft view. Right? Now, if the above is right, then I have a question for people: HOW IN THE HELL DO WE HAVE TIME FOR THOUSANDS OF THREADS TO HIT THAT ONE PAGE? That just sounds really sketchy to me. Even if all those thousands of threads are runnable, we need to schedule into them just to get them to wait on that one page. So that sounds really quite odd when migration is supposed to hold the page lock for a relatively short time and get out. Don't you agree? Which is why I started thinking of what the hell could go on for that long wait-queue to happen. One thing that strikes me is that the way wait_on_page_bit() works is that it will NOT wait until the next bit clearing, it will wait until it actively *sees* the page bit being clear. Now, work with me on that. What's the difference? What we could have is some bad NUMA balancing pattern that actually has a page that everybody touches. And hey, we pretty much know that everybody touches that page, since people get stuck on that wait-queue, right? And since everybody touches it, as a result everybody eventually thinks that page should be migrated to their NUMA node. But for all we know, the migration keeps on failing, because one of the points of that "lock page - try to move - unlock page" is that *TRY* in "try to move". There's a number of things that makes it not actually migrate. Like not being movable, or failing to isolate the page, or whatever. So we could have some situation where we end up locking and unlocking the page over and over again (which admittedly is already a sign of something wrong in the NUMA balancing, but that's a separate issue). And if we get into that situation, where everybody wants that one hot page, what happens to the waiters? One of the thousands of waiters is unlucky (remember, this argument started with the whole "you shouldn't get that many waiters on one single page that isn't even locked for that long"), and goes: (a) Oh, the page is locked, I will wait for the lock bit to clear (b) go to sleep (c) the migration fails, the lock bit is cleared, the waiter is woken up but doesn't get the CPU immediately, and one of the other *thousands* of threads decides to also try to migrate (see above), (d) the guy waiting for the lock bit to clear will see the page "still" locked (really just "locked again") and continue to wait. In the meantime, one of the other threads happens to be unlucky, also hits the race, and now we have one more thread waiting for that page lock. It keeps getting unlocked, but it also keeps on getting locked, and so the queue can keep growing. See where I'm going here? I think it's really odd how *thousands* of threads can hit that locked window that is supposed to be pretty small. But I think it's much more likely if we have some kind of repeated event going on. So I'm starting to think that part of the problem may be how stupid that "wait_for_page_bit_common()" code is. It really shouldn't wait until it sees that the bit is clear. It could have been cleared and then re-taken. And honestly, we actually have extra code for that "let's go round again". That seems pointless. If the bit has been cleared, we've been woken up, and nothing else would have done so anyway, so if we're not interested in locking, we're simply *done* after we've done the "io_scheduler()". So I propose testing the attached trivial patch. It may not do anything at all. But the existing code is actually doing extra work just to be fragile, in case the scenario above can happen. Comments? 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