From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Howells <dhowells@redhat.com>,
kernel test robot <oliver.sang@intel.com>,
oe-lkp@lists.linux.dev, lkp@intel.com,
linux-kernel@vger.kernel.org,
Christian Brauner <brauner@kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
Christian Brauner <christian@brauner.io>,
Matthew Wilcox <willy@infradead.org>,
David Laight <David.Laight@aculab.com>,
ying.huang@intel.com, feng.tang@intel.com, fengwei.yin@intel.com,
linux-toolchains ML <linux-toolchains@vger.kernel.org>
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression
Date: Fri, 17 Nov 2023 12:44:21 +0100 [thread overview]
Message-ID: <20231117114421.GCZVdSFZ7DKtBol821@fat_crate.local> (raw)
In-Reply-To: <CAHk-=wjGQh3ucZFmFR0evbKu2OyEuue-bOjsrnCvxSQdj8x6aw@mail.gmail.com>
Might as well Cc toolchains...
On Thu, Nov 16, 2023 at 11:48:18AM -0500, Linus Torvalds wrote:
> Hmm. I know about the '-mstringop-strategy' flag because of the fairly
> recently discussed bug where gcc would create a byte-by-byte copy in
> some crazy circumstances with the address space attributes:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111657
I hear those stringop strategy heuristics are interesting. :)
> But I incorrectly thought that "-mstringop-strategy=libcall" would
> then *always* do library calls.
That's how I understood it too. BUT, reportedly, small and known sizes
are still optimized, which is exactly what we want.
> So I decided to test, and that shows that gcc still ends up doing the
> "expand small constant size copies inline" even with that option, and
> doesn't force library calls for those cases.
And you've confirmed it.
> IOW, my assumption was just broken, and using
> "-mstringop-strategy=libcall" may well be the right thing to do.
And here's where I'm wondering whether we should enable it for x86 only
or globally. I think globally because those stringop heuristics happen,
AFAIU, in the general optimization stage and thus target agnostic.
> Of course, it's also possible that with all the function call overhead
> introduced by the CPU mitigations on older CPU's, we should just say
> "rep movsb" is always correct - if you have a new CPU with FSRM it's
> good, and if you have an old CPU it's no worse than the horrendous CPU
> mitigation overhead for function call/returns.
Yeah, I think we should measure the libcall thing and then try to get
the inlined "rep movsb" working and see which one is better. You do have
a point about that RET overhead after each CALL.
> I really hate the mitigations. Oh well.
Tell me about it.
> Ayway, maybe your patch is the RightThing(tm). Or maybe we should use
> 'rep_byte' instead of 'libcall'. Who knows..
Yeah, lemme keep playing with this.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next parent reply other threads:[~2023-11-17 11:44 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <202311061616.cd495695-oliver.sang@intel.com>
[not found] ` <3865842.1700061614@warthog.procyon.org.uk>
[not found] ` <CAHk-=whM-cEwAsLtKsf5dYwV7nDTaRv1bUKLVBstMAQBug24uQ@mail.gmail.com>
[not found] ` <CAHk-=wjCUckvZUQf7gqp2ziJUWxVpikM_6srFdbcNdBJTxExRg@mail.gmail.com>
[not found] ` <CAHk-=wjhs6uuedgz-7HbcPtirEq+vvjJBY-M2zyteJwBhOMZhg@mail.gmail.com>
[not found] ` <20231115190938.GGZVUXcuUjI3i1JRAB@fat_crate.local>
[not found] ` <CAHk-=wh0TcXyGmKHfs+Xe=5Sd5bNn=NNV9CEtOy_tbyHAAmk9g@mail.gmail.com>
[not found] ` <20231116154406.GDZVY4xmFvRQt0wGGE@fat_crate.local>
[not found] ` <CAHk-=wjGQh3ucZFmFR0evbKu2OyEuue-bOjsrnCvxSQdj8x6aw@mail.gmail.com>
2023-11-17 11:44 ` Borislav Petkov [this message]
2023-11-17 12:09 ` [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression Jakub Jelinek
2023-11-17 12:18 ` Borislav Petkov
2023-11-17 13:09 ` David Laight
2023-11-17 13:36 ` Linus Torvalds
2023-11-17 15:20 ` David Laight
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