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Message-ID: <6952b581-68d2-476b-9f7e-e9746c8c0473@efficios.com>
Date: Wed, 13 Mar 2024 18:06:24 -0400
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Subject: Re: [RFC PATCH 0/2] Introduce serialized smp_call_function APIs
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To: Avi Kivity <avi@scylladb.com>, Peter Oskolkov <posk@google.com>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
 "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
 Boqun Feng <boqun.feng@gmail.com>, Andrew Hunter <ahh@google.com>,
 Maged Michael <maged.michael@gmail.com>, gromer@google.com,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Paul Mackerras <paulus@samba.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Linus Torvalds <torvalds@linux-foundation.org>
References: <20240313205622.2179659-1-mathieu.desnoyers@efficios.com>
 <7c1860d783e0f317160069bf77d8fafdf01a9f97.camel@scylladb.com>
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <7c1860d783e0f317160069bf77d8fafdf01a9f97.camel@scylladb.com>
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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