From: Andy Lutomirski <luto@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>,
"Michael S. Tsirkin" <mst@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
Davidlohr Bueso <dbueso@suse.de>,
Peter Zijlstra <peterz@infradead.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
virtualization <virtualization@lists.linux-foundation.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()
Date: Tue, 12 Jan 2016 12:30:46 -0800 [thread overview]
Message-ID: <56956276.1090705@kernel.org> (raw)
In-Reply-To: <CA+55aFwqgUQYVbVXLw1=LL6Gs=kXqhkx0tUZOdXnWbqCMdWfXg@mail.gmail.com>
On 01/12/2016 09:20 AM, Linus Torvalds wrote:
> On Tue, Jan 12, 2016 at 5:57 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> #ifdef xchgrz
>> /* same as xchg but poking at gcc red zone */
>> #define barrier() do { int ret; asm volatile ("xchgl %0, -4(%%" SP ");": "=r"(ret) :: "memory", "cc"); } while (0)
>> #endif
>
> That's not safe in general. gcc might be using its redzone, so doing
> xchg into it is unsafe.
>
> But..
>
>> Is this a good way to test it?
>
> .. it's fine for some basic testing. It doesn't show any subtle
> interactions (ie some operations may have different dynamic behavior
> when the write buffers are busy etc), but as a baseline for "how fast
> can things go" the stupid raw loop is fine. And while the xchg into
> the redzoen wouldn't be acceptable as a real implementation, for
> timing testing it's likely fine (ie you aren't hitting the problem it
> can cause).
I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even
64) was better because it avoided stomping on very-likely-to-be-hot
write buffers.
--Andy
next prev parent reply other threads:[~2016-01-12 20:31 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-27 19:53 [PATCH -tip 0/4] A few updates around smp_store_mb() Davidlohr Bueso
2015-10-27 19:53 ` [PATCH 1/4] arch,cmpxchg: Remove tas() definitions Davidlohr Bueso
2015-12-04 12:01 ` [tip:locking/core] locking/cmpxchg, arch: " tip-bot for Davidlohr Bueso
2015-10-27 19:53 ` [PATCH 2/4] arch,barrier: Use smp barriers in smp_store_release() Davidlohr Bueso
2015-10-27 20:03 ` Davidlohr Bueso
2015-12-04 12:01 ` [tip:locking/core] lcoking/barriers, arch: " tip-bot for Davidlohr Bueso
2015-10-27 19:53 ` [PATCH 3/4] x86,asm: Re-work smp_store_mb() Davidlohr Bueso
2015-10-27 21:33 ` Linus Torvalds
2015-10-27 22:01 ` Davidlohr Bueso
2015-10-27 22:37 ` Peter Zijlstra
2015-10-28 19:49 ` Davidlohr Bueso
2015-11-02 20:15 ` Davidlohr Bueso
2015-11-03 0:06 ` Linus Torvalds
2015-11-03 1:36 ` Davidlohr Bueso
2016-01-12 13:57 ` Michael S. Tsirkin
2016-01-12 17:20 ` Linus Torvalds
2016-01-12 17:45 ` Michael S. Tsirkin
2016-01-12 18:04 ` Linus Torvalds
2016-01-12 20:30 ` Andy Lutomirski [this message]
2016-01-12 20:54 ` Linus Torvalds
2016-01-12 20:59 ` Andy Lutomirski
2016-01-12 21:37 ` Linus Torvalds
2016-01-12 22:14 ` Michael S. Tsirkin
2016-01-13 16:20 ` Michael S. Tsirkin
2016-01-12 22:21 ` Michael S. Tsirkin
2016-01-12 22:55 ` H. Peter Anvin
2016-01-12 23:24 ` Linus Torvalds
2016-01-13 16:17 ` Borislav Petkov
2016-01-13 16:25 ` Michael S. Tsirkin
2016-01-13 16:33 ` Borislav Petkov
2016-01-13 16:42 ` Michael S. Tsirkin
2016-01-13 16:53 ` Borislav Petkov
2016-01-13 17:00 ` Michael S. Tsirkin
2016-01-13 18:38 ` Linus Torvalds
2015-10-27 19:53 ` [PATCH 4/4] doc,smp: Remove ambiguous statement in smp_store_mb() Davidlohr Bueso
2015-12-04 12:01 ` [tip:locking/core] locking/barriers, arch: Remove ambiguous statement in the smp_store_mb() documentation tip-bot for Davidlohr Bueso
2015-10-27 23:27 ` [PATCH 1/4] arch,cmpxchg: Remove tas() definitions David Howells
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