From: Steven Rostedt <rostedt@goodmis.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Christoph Lameter <cl@linux.com>,
Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Jesper Dangaard Brouer <brouer@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off
Date: Thu, 15 Jan 2015 20:30:45 -0500 [thread overview]
Message-ID: <20150115203045.00e9fb73@grimm.local.home> (raw)
In-Reply-To: <20150115171634.685237a4.akpm@linux-foundation.org>
On Thu, 15 Jan 2015 17:16:34 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:
> > I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free
> > in CONFIG_PREEMPT. (14.821 ns -> 14.049 ns)
>
> I'm surprised. preempt_disable/enable are pretty fast. I wonder why
> this makes a measurable difference. Perhaps preempt_enable()'s call
> to preempt_schedule() added pain?
profiling function tracing I discovered that accessing preempt_count
was actually quite expensive, even just to read. But it may not be as
bad since Peter Zijlstra converted preempt_count to a per_cpu variable.
Although, IIRC, the perf profiling showed the access to the %gs
register was where the time consuming was happening, which is what
I believe per_cpu variables still use.
-- Steve
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next prev parent reply other threads:[~2015-01-16 1:30 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-01-15 7:40 Joonsoo Kim
2015-01-15 7:40 ` [PATCH v2 2/2] mm: don't use compound_head() in virt_to_head_page() Joonsoo Kim
2015-01-16 1:16 ` Andrew Morton
2015-01-16 3:30 ` Christoph Lameter
2015-01-19 4:20 ` 答复: " Zhang, Yanfei
2015-01-19 6:16 ` Joonsoo Kim
2015-01-15 8:10 ` [PATCH v2 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off Jesper Dangaard Brouer
2015-01-16 1:16 ` Andrew Morton
2015-01-16 1:30 ` Steven Rostedt [this message]
2015-01-16 3:27 ` Christoph Lameter
2015-01-16 3:51 ` Steven Rostedt
2015-01-16 3:57 ` Christoph Lameter
2015-01-16 4:07 ` Steven Rostedt
2015-01-16 13:40 ` Eric Dumazet
2015-01-16 16:37 ` Steven Rostedt
2015-01-16 4:04 ` Steven Rostedt
2015-01-16 3:28 ` Christoph Lameter
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