From: Manfred Spraul <manfred@colorfullife.com>
To: dipankar@in.ibm.com
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>,
Oleg Nesterov <oleg@tv-sign.ru>,
linux-kernel@vger.kernel.org, Linus Torvalds <torvalds@osdl.org>,
Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH 4/5] rcu: join rcu_ctrlblk and rcu_state
Date: Tue, 10 Jan 2006 21:44:56 +0100 [thread overview]
Message-ID: <43C41CC8.8000203@colorfullife.com> (raw)
In-Reply-To: <20060110180954.GA5387@in.ibm.com>
[I haven't read the diff, just a short comment]
Dipankar Sarma wrote:
>rcu_state came over from Manfred's RCU_HUGE patch IIRC. I don't
>think it is necessary to allocate rcu_state separately in the
>current mainline RCU code. So, the patch looks OK to me, but
>Manfred might see something that I am not seeing.
>
>
>
The two-level rcu code was never merged, I still plan to clean it up.
But the idea of splitting the control block and the state is used in the
current code:
- __rcu_pending() is the hot path, it only performs a read access to
rcu_ctrlblk.
- write accesses to the rcu_ctrlblk are really rare, they only happen
when a new batch is started. Especially: independant from the number of
cpus.
Write access to the rcu_state are common:
- each cpu must write once in each cycle to update it's cpu mask.
- The last cpu then completes the quiescent cycle.
The idea is that rcu_state is more or less write-only and rcu_state is
read-only. Theoretically, rcu_state could be shared in all cpus caches,
and there will be only one invalidate when a new batch is started. Thus
no cacheline trashing due to rcu_pending calls.
I think it would be safer to keep the two state counters in a separate
cacheline from the spinlock and the cpu mask, but I don't have any hard
numbers. IIRC the problems with the large SGI systems disappered, and
everyone was happy. No real benchmark comparisons were made.
--
Manfred
next prev parent reply other threads:[~2006-01-10 20:45 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-08 19:19 [PATCH 4/5] rcu: join rcu_ctrlblk and rcu_state Oleg Nesterov
2006-01-10 0:28 ` Paul E. McKenney
2006-01-10 0:43 ` Linus Torvalds
2006-01-10 2:54 ` Paul E. McKenney
2006-01-10 10:02 ` Srivatsa Vaddagiri
2006-01-10 18:09 ` Dipankar Sarma
2006-01-10 20:44 ` Manfred Spraul [this message]
2006-01-10 21:51 ` Paul E. McKenney
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