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

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Message-ID: <6952b581-68d2-476b-9f7e-e9746c8c0473@efficios.com>
Date: Wed, 13 Mar 2024 18:06:24 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Avi Kivity <avi@...lladb.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>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/2] Introduce serialized smp_call_function APIs

On 2024-03-13 17:14, Avi Kivity wrote:
> 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.
>>
>> This series applies on top of v6.8.
>>
> 
> 
> I see this doesn't restore scaling of MEMBARRIER_CMD_PRIVATE_EXPEDITED, 
> which I use (and wasn't aware was broken).

It's mainly a mitigation for IPI Storming: CVE-2024-26602 disclosed
as part of [1].

> 
> 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.

Note that if you don't use membarrier private expedited too heavily,
you should not notice any difference. But nevertheless I would be
interested to hear about any regression on performance of real
workloads resulting from commit 944d5fe50f3f.

Thanks,

Mathieu

[1] https://www.vusec.net/projects/ghostrace/

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com