linux-kernel - Re: [PATCH v2] rcu: Reduce synchronize_rcu() delays when all wait heads are in use

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Date: Thu, 4 Apr 2024 23:42:38 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Cc: paulmck@...nel.org, joel@...lfernandes.org, urezki@...il.com,
	josh@...htriplett.org, boqun.feng@...il.com, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
	qiang.zhang1211@...il.com, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, neeraj.upadhyay@...nel.org
Subject: Re: [PATCH v2] rcu: Reduce synchronize_rcu() delays when all wait
 heads are in use

Le Wed, Apr 03, 2024 at 04:22:12PM +0530, Neeraj Upadhyay a écrit :
> When all wait heads are in use, which can happen when
> rcu_sr_normal_gp_cleanup_work()'s callback processing
> is slow, any new synchronize_rcu() user's rcu_synchronize
> node's processing is deferred to future GP periods. This
> can result in long list of synchronize_rcu() invocations
> waiting for full grace period processing, which can delay
> freeing of memory. Mitigate this problem by using first
> node in the list as wait tail when all wait heads are in use.
> While methods to speed up callback processing would be needed
> to recover from this situation, allowing new nodes to complete
> their grace period can help prevent delays due to a fixed
> number of wait head nodes.
> 
> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@....com>

Looking at it again, I'm not sure if it's a good idea to
optimize the thing that far. It's already a tricky state machine
to review and the workqueue has SR_NORMAL_GP_WAIT_HEAD_MAX - 1 = 4
grace periods worth of time to execute. Such a tense situation may
happen of course but, should we really work around that?

I let you guys judge. In the meantime, I haven't found correctness
issues:

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks.