Re: [RFC PATCH 0/2] Introduce serialized smp_call_function APIs

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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