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

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Date:   Wed, 22 Nov 2023 21:13:43 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Charlie Jenkins <charlie@...osinc.com>,
        Andrea Parri <parri.andrea@...il.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>, rehn@...osinc.com,
        paulmck@...nel.org, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, mmaas@...gle.com, hboehm@...gle.com,
        striker@...ibm.com
Subject: Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

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@rivosinc.com/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