From: David Rientjes <rientjes@google.com>
To: Kautuk Consul <consul.kautuk@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Joe Perches <joe@perches.com>,
Minchan Kim <minchan.kim@gmail.com>,
David Vrabel <david.vrabel@citrix.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] vmalloc: purge_fragmented_blocks: Acquire spinlock before reading vmap_block
Date: Wed, 7 Dec 2011 23:18:59 -0800 (PST) [thread overview]
Message-ID: <alpine.DEB.2.00.1112072314140.28419@chino.kir.corp.google.com> (raw)
In-Reply-To: <CAFPAmTSJDXD1KNVBUz75yN_CeCT9f_+W9CaRNN467LSyCD+WXg@mail.gmail.com>
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On Thu, 8 Dec 2011, Kautuk Consul wrote:
> > In the original code, if the if-clause fails, the lock is only then taken
> > and the exact same test occurs again while protected. A If the test now
> > fails, the lock is immediately dropped. A A branch here is faster than a
> > contented spinlock.
>
> But, if there is some concurrent change happening to vb->free and
> vb->dirty, dont you think
> that it will continue and then go to the next vmap_block ?
>
> If yes, then it will not be put into the purge list.
>
That's intentional as an optimization, we don't care if
vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS
would speculatively be true after we grab vb->lock, we'll have to purge it
next time instead. We certainly don't want to grab vb->lock for blocks
that aren't candidates, so this optimization is a singificant speedup.
next prev parent reply other threads:[~2011-12-08 7:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-08 7:02 Kautuk Consul
2011-12-08 7:07 ` David Rientjes
2011-12-08 7:13 ` Kautuk Consul
2011-12-08 7:18 ` David Rientjes [this message]
2011-12-08 7:26 ` Kautuk Consul
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