From: Waiman Long <longman@redhat.com>
To: 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"
<linux-arm-kernel@lists.infradead.org>,
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the arch/x86 maintainers <x86@kernel.org>,
Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Andrew Morton <akpm@linux-foundation.org>,
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 13:23:55 -0400 [thread overview]
Message-ID: <15530d24-51fd-579d-2b1a-2ca98487f63f@redhat.com> (raw)
In-Reply-To: <CAHk-=wikkO-1f1=FEOEzkSnaDg3yJLP=4Vd59UCuLBztFd0KOw@mail.gmail.com>
On 03/22/2019 01:01 PM, Linus Torvalds wrote:
> On Fri, Mar 22, 2019 at 7:30 AM Waiman Long <longman@redhat.com> wrote:
>> 19 files changed, 133 insertions(+), 930 deletions(-)
> Lovely. And it all looks sane to me.
>
> So ack.
>
> The only comment I have is about __down_read_trylock(), which probably
> isn't critical enough to actually care about, but:
>
>> +static inline int __down_read_trylock(struct rw_semaphore *sem)
>> +{
>> + long tmp;
>> +
>> + while ((tmp = atomic_long_read(&sem->count)) >= 0) {
>> + if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
>> + tmp + RWSEM_ACTIVE_READ_BIAS)) {
>> + return 1;
>> + }
>> + }
>> + return 0;
>> +}
> So this seems to
>
> (a) read the line early (the whole cacheline in shared state issue)
>
> (b) read the line again unnecessarily in the while loop
>
> Now, (a) might be explained by "well, maybe we do trylock even with
> existing readers", although I continue to think that the case we
> should optimize for is simply the uncontended one, where we don't even
> have multiple readers.
>
> But (b) just seems silly.
>
> So I wonder if it shouldn't just be
>
> long tmp = 0;
>
> do {
> long new = atomic_long_cmpxchg_acquire(&sem->count, tmp,
> tmp + RWSEM_ACTIVE_READ_BIAS);
> if (likely(new == tmp))
> return 1;
> tmp = new;
> } while (tmp >= 0);
> return 0;
>
> which would seem simpler and solve both issues. Hmm?
>
> But honestly, I didn't check what our uses of down_read_trylock() look
> like. We have more of them than I expected, and I _think_ the normal
> case is the "nobody else holds the lock", but that's just a gut
> feeling.
>
> 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.
>
> NOTE! The above code snippet is absolutely untested, and might be
> completely wrong. Take it as a "something like this" rather than
> anything else.
>
> Linus
As you have noticed already, this patch is just for moving code around
without changing it. I optimize __down_read_trylock() in patch 3.
Cheers,
Longman
next prev parent reply other threads:[~2019-03-22 17:24 UTC|newest]
Thread overview: 20+ 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 [this message]
2019-03-22 19:30 ` Davidlohr Bueso
2019-03-22 20:27 ` Waiman Long
2019-04-03 10:44 ` [tip:locking/core] " tip-bot for Waiman Long
2019-04-03 12:47 ` Peter Zijlstra
2019-04-03 12:55 ` tip-bot for Waiman Long
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
2019-04-03 10:44 ` [tip:locking/core] " tip-bot for Waiman Long
2019-04-03 12:55 ` tip-bot for Waiman Long
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
2019-04-03 10:45 ` [tip:locking/core] " tip-bot for Waiman Long
2019-04-03 12:56 ` tip-bot for Waiman Long
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