Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

[Mathieu Desnoyers]

On 2023-11-22 20:07, Charlie Jenkins wrote:
> On Thu, Nov 09, 2023 at 08:24:58PM +0100, Andrea Parri wrote:
> > Mathieu, all,
> >
> > Sorry for the delay,
> >
> > > AFAIR this patch implements sync_core_before_usermode which gets used by
> > > membarrier_mm_sync_core_before_usermode() to handle the uthread->kthread->uthread
> > > case. It relies on switch_mm issuing a core serializing instruction as well.
> > >
> > > Looking at RISC-V switch_mm(), I see that switch_mm() calls:
> > >
> > >    flush_icache_deferred(next, cpu);
> > >
> > > which only issues a fence.i if a deferred icache flush was required. We're
> > > missing the part that sets the icache_stale_mask cpumask bits when a
> > > MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is invoked.
> >
> > [...]
> >
> > > The only part where I think you may want to keep some level of deferred
> > > icache flushing as you do now is as follows:
> > >
> > > - when membarrier MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is invoked,
> > >    call a new architecture hook which sets cpumask bits in the mm context
> > >    that tells the next switch_mm on each cpu to issue fence.i for that mm.
> > > - keep something like flush_icache_deferred as you have now.
> > >
> > > Otherwise, I fear the overhead of a very expensive fence.i would be too
> > > much when processes registering with MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE
> > > and start doing fence.i on each and every switch_mm().
> > >
> > > So you'd basically rely on membarrier to only issue IPIs to the CPUs which are
> > > currently running threads belonging to the mm, and handle the switch_mm with
> > > the sync_core_before_usermode() for uthread->kthread->uthread case, and implement
> > > a deferred icache flush for the typical switch_mm() case.
> >
> > I've (tried to) put this together and obtained the two patches reported below.
> > Please let me know if this aligns with your intentions and/or there's interest
> > in a proper submission.

>
> This looks good to me, can you send out a non-RFC? I just sent out
> patches to support userspace fence.i:
> 
https://lore.kernel.org/linux-riscv/20231122-fencei-v1-0-bec0811cb212@xxxxxxxxxxxx/T/#t.
>
> - Charlie
>

Hi Andrea,

Yes, please send those as non-RFC patches. They align well with my 
intentions.

Thanks!

Mathieu


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