linux-kernel - Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

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Date:   Thu, 5 Oct 2017 15:54:38 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Avi Kivity <avi@...lladb.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        gromer <gromer@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited
 private command

----- On Oct 5, 2017, at 8:22 AM, Avi Kivity avi@...lladb.com wrote:

> On 10/05/2017 07:23 AM, Nicholas Piggin wrote:
>> On Wed,  4 Oct 2017 14:37:53 -0700
>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
>>
>>> From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>>>
>>> Provide a new command allowing processes to register their intent to use
>>> the private expedited command.
>>>
>>> This allows PowerPC to skip the full memory barrier in switch_mm(), and
>>> only issue the barrier when scheduling into a task belonging to a
>>> process that has registered to use expedited private.
>>>
>>> Processes are now required to register before using
>>> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>>>
>>> Changes since v1:
>>> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>>>    powerpc membarrier_arch_sched_in(), given that we want to specifically
>>>    check the next thread state.
>>> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
>>> - Use task_thread_info() to pass thread_info from task to
>>>    *_ti_thread_flag().
>>>
>>> Changes since v2:
>>> - Move membarrier_arch_sched_in() call to finish_task_switch().
>>> - Check for NULL t->mm in membarrier_arch_fork().
>>> - Use membarrier_sched_in() in generic code, which invokes the
>>>    arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>>>    build on PowerPC.
>>> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>>>    allnoconfig build on PowerPC.
>>> - Build and runtime tested on PowerPC.
>>>
>>> Changes since v3:
>>> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>>>    field on fork.
>>> - powerpc: test thread flag instead of reading
>>>    membarrier_private_expedited in membarrier_arch_fork().
>>> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>>>    from kernel thread, since mmdrop() implies a full barrier.
>>> - Set membarrier_private_expedited to 1 only after arch registration
>>>    code, thus eliminating a race where concurrent commands could succeed
>>>    when they should fail if issued concurrently with process
>>>    registration.
>>> - Use READ_ONCE() for membarrier_private_expedited field access in
>>>    membarrier_private_expedited. Matches WRITE_ONCE() performed in
>>>    process registration.
>>>
>>> Changes since v4:
>>> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>>>    from Nicholas Piggin.
>> For now, the powerpc approach is okay by me. I plan to test
>> others (e.g., taking runqueue locks) on larger systems, but that can
>> be sent as an incremental patch at a later time.
>>
>> The main thing I would like is for people to review the userspace API.
>>
> 
> As a future satisfied user of the expedited private membarrier syscall,
> I am happy with the change.

Thanks Avi for your input on the userspace API.

Mathieu


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