linux-kernel - Re: [RFC PATCH 0/2] Introduce serialized smp_call

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Message-ID: <96325519f8f05d7289ec9b0af4e89da28c432e4f.camel@scylladb.com>
Date: Wed, 13 Mar 2024 23:58:44 +0200
From: Avi Kivity <avi@...lladb.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Oskolkov
	 <posk@...gle.com>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, 
 "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Boqun Feng
 <boqun.feng@...il.com>, Andrew Hunter <ahh@...gle.com>,  Maged Michael
 <maged.michael@...il.com>, gromer@...gle.com, Benjamin Herrenschmidt
 <benh@...nel.crashing.org>,  Paul Mackerras <paulus@...ba.org>, Michael
 Ellerman <mpe@...erman.id.au>
Subject: Re: [RFC PATCH 0/2] Introduce serialized smp_call_function APIs

[resend in plain text for the list]

On Wed, 2024-03-13 at 16:56 -0400, Mathieu Desnoyers wrote:
> commit 944d5fe50f3f ("sched/membarrier: reduce the ability to hammer
> on sys_membarrier")
> introduces a mutex over all membarrier operations to reduce its
> ability
> to slow down the rest of the system.
> 
> This RFC series has two objectives:
> 
> 1) Move this mutex to the smp_call_function APIs so other system
> calls
>    using smp_call_function IPIs are limited in the same way,
> 
> 2) Restore scalability of MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ with
>    MEMBARRIER_CMD_FLAG_CPU, which targets specific CPUs with IPIs.
>    This may or may not be useful, and I would welcome benchmarks from
>    users of this feature to figure out if this is worth it.
> 

I see this doesn't restore scaling of MEMBARRIER_CMD_PRIVATE_EXPEDITED,
which I use (and wasn't aware was broken).

I don't have comments on the patches, but do have ideas on how to work
around the problem in Seastar. So this was a useful heads-up for me.