From: Waiman Long <longman@redhat.com>
To: Davidlohr Bueso <dave@stgolabs.net>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
"linux-alpha@vger.kernel.org" <linux-alpha@vger.kernel.org>,
"linux-alpha@vger.kernel.org"
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Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files
Date: Fri, 22 Mar 2019 16:27:26 -0400 [thread overview]
Message-ID: <45315aa9-f006-c4ca-705f-71d6c2650508@redhat.com> (raw)
In-Reply-To: <20190322193010.azb7rmmmaclhal35@linux-r8p5>
On 03/22/2019 03:30 PM, Davidlohr Bueso wrote:
> On Fri, 22 Mar 2019, Linus Torvalds wrote:
>> Some of them _might_ be performance-critical. There's the one on
>> mmap_sem in the fault handling path, for example. And yes, I'd expect
>> the normal case to very much be "no other readers or writers" for that
>> one.
>
> Yeah, the mmap_sem case in the fault path is really expecting an unlocked
> state. To the point that four archs have added branch predictions, ie:
>
> 92181f190b6 (x86: optimise x86's do_page_fault (C entry point for the
> page fault path))
> b15021d994f (powerpc/mm: Add a bunch of (un)likely annotations to
> do_page_fault)
>
> And using PROFILE_ANNOTATED_BRANCHES shows pretty clearly:
> (without resetting the counters)
>
> correct incorrect % Function File Line
> ------- --------- - -------- ---- ----
> 4603685 34 0 do_user_addr_fault fault.c 1416
> (bootup)
> 382327745 449 0 do_user_addr_fault fault.c
> 1416 (kernel build)
> 399446159 461 0 do_user_addr_fault fault.c
> 1416 (redis benchmark)
>
> It would probably wouldn't harm doing the unlikely() for all archs, or
> alternatively, add likely() to the atomic_long_try_cmpxchg_acquire in
> patch 3 and do it implicitly but maybe that would be less flexible(?)
>
> Thanks,
> Davidlohr
I had used the my lock event counting code to count the number of
contended and uncontended trylocks. I tested both bootup and kernel
build. I think I saw less than 1% were contended, the rests were all
uncontended. That is similar to what you got. I thought I had sent the
data out previously, but I couldn't find the email. That was the main
reason why I took Linus' suggestion to optimize it for the uncontended case.
Thanks,
Longman
next prev parent reply other threads:[~2019-03-22 20:27 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 14:30 [PATCH v5 0/3] locking/rwsem: Rwsem rearchitecture part 0 Waiman Long
2019-03-22 14:30 ` [PATCH v5 1/3] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-03-22 17:01 ` Linus Torvalds
2019-03-22 17:23 ` Waiman Long
2019-03-22 19:30 ` Davidlohr Bueso
2019-03-22 20:27 ` Waiman Long [this message]
2019-03-22 14:30 ` [PATCH v5 2/3] locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all archs Waiman Long
2019-03-22 17:07 ` Linus Torvalds
2019-03-22 14:30 ` [PATCH v5 3/3] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-03-22 17:08 ` Linus Torvalds
2019-03-22 17:25 ` Russell King - ARM Linux admin
2019-03-22 17:41 ` Waiman Long
2019-03-25 15:25 ` Christophe Leroy
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