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

[Mathieu Desnoyers]

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