Bug 169510

Summary:

REGRESSION(r213645): It made JSC tests super slow and timeout on AArch64 Linux

Product:

WebKit

Reporter:

Csaba Osztrogonác <ossy>

Component:

JavaScriptCore

Assignee:

Zan Dobersek <zan>

Status:

RESOLVED FIXED

Severity:

Blocker

CC:

benjamin, buildbot, cdumez, clopez, cmarcelo, dbates, fpizlo, mark.lam, ossy, saam, zan

Priority:

Version:

WebKit Nightly Build

Hardware:

All

OS:

Linux

Bug Depends on:

Bug Blocks:

108645, 169300

Attachments:

Description	Flags
Patch	none

Csaba Osztrogonác

Reported 2017-03-11 00:46:59 PST

before r213645: - https://build.webkit.org/builders/JSCOnly%20Linux%20AArch64%20Release/builds/4895 - runtime: 1h 15mins - only 7 failures after r213645: - https://build.webkit.org/builders/JSCOnly%20Linux%20AArch64%20Release/builds/4950 - runtime: 10h 30 mins (stopped after it, full test runtime would have taken days) - zillion timeouts Note, that https://trac.webkit.org/changeset/213645 broke the build too which was fixed in https://trac.webkit.org/changeset/213705. But I tested r213645 with the buildfix directly and it showed that r213645 caused this serious regression. Is there any way to disable this super slow new feature on Linux? I don't and won't have to debug what caused this regeression in your patch.

Attachments
Patch (2.31 KB, patch) 2017-04-03 12:03 PDT, Zan Dobersek	no flags	Details Formatted Diff Diff
View All Add attachment proposed patch, testcase, etc.

Csaba Osztrogonác

Comment 1 2017-03-11 05:58:11 PST

I disabled it on Linux in https://trac.webkit.org/changeset/213758. Please enable it again once somebody fixed this regression.

Filip Pizlo

Comment 2 2017-03-11 07:44:50 PST

We should figure out what's going on here. This is probably not a Linux versus Darwin thing but a CPU model thing. Here's a possible hint: http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150921/300953.html It looks like in LLVM, they have to emit clrex when exiting ldx/stx (LL/SC) loops because "some uarchs" need it for "performance". I tried emitting clrex, and on my hardware, it made no difference. I think in our jargon, "clrex" should be called "WTF::clearLink()", since we use the old computer science terminology for this (load link, store conditional) rather than the ARM terminology (reservations). The way this would look like is that you'd want Atomic<>::transaction() to call clearLink() whenever it returns after having done a loadLink() but before the storeCond() - basically anytime that we would otherwise be leaving the link (the reservation) in place. If this works then just as transaction() abstracts LL/SC and CAS behind prepare/attempt, you probably want a giveUp() method that is a wrapper for clearLink() on LL_SC platforms and a no-op on CAS platforms. You probably want clearLink() to only be declared when HAVE(LL_SC) just as loadLink()/storeCond() are only declared when HAVE(LL_SC). I think it's OK to do clearLink() on all ARM64s, if it helps your perf. If this works, then can you also put a FIXME in B3LowerToAir's code that emits loadlink/storecond, and MacroAssembler's loadLink/storeCond code, and link it to a bug? Once I get around to implementing SharedArrayBuffer atomics, I'll definitely need to use MacroAssembler's and B3's LL/SC. It won't be possible to easily get around this.

Csaba Osztrogonác

Comment 3 2017-03-13 05:21:56 PDT

(In reply to comment #2) > We should figure out what's going on here. This is probably not a Linux > versus Darwin thing but a CPU model thing. ... Of course, yes and thanks for the detailed info. But as I said I don't and won't have to do it myself in the (near) future. If somebody else is interested in debugging this issue, feel free to pick it up.

Zan Dobersek

Comment 4 2017-04-03 12:03:42 PDT

Created attachment 306092 [details] Patch

Build Bot

Comment 5 2017-04-03 12:05:00 PDT

Attachment 306092 [details] did not pass style-queue: ERROR: Source/WTF/wtf/Atomics.h:193: Extra space before ( in function call [whitespace/parens] [4] ERROR: Source/WTF/wtf/Atomics.h:213: Extra space before ( in function call [whitespace/parens] [4] Total errors found: 2 in 3 files If any of these errors are false positives, please file a bug against check-webkit-style.

Saam Barati

Comment 6 2017-04-10 19:23:35 PDT

Comment on attachment 306092 [details] Patch r=me

Zan Dobersek

Comment 7 2017-04-11 01:27:32 PDT

Comment on attachment 306092 [details] Patch Clearing flags on attachment: 306092 Committed r215220: <http://trac.webkit.org/changeset/215220>

Zan Dobersek

Comment 8 2017-04-11 01:27:36 PDT

All reviewed patches have been landed. Closing bug.

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