linux-kernel - Re: [PATCH v2] sched/membarrier: Fix redundant load of membarrier

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Message-ID: <87ed3yebei.fsf@mpe.ellerman.id.au>
Date: Wed, 30 Oct 2024 10:29:09 +1100
From: Michael Ellerman <mpe@...erman.id.au>
To: "Nysal Jan K.A." <nysal@...ux.ibm.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>
Cc: Segher Boessenkool <segher@...nel.crashing.org>, Stephen Rothwell
 <sfr@...b.auug.org.au>, Peter Zijlstra <peterz@...radead.org>,
 linuxppc-dev@...ts.ozlabs.org, "Nysal Jan K.A." <nysal@...ux.ibm.com>,
 Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, "Liam R.
 Howlett" <Liam.Howlett@...cle.com>, Mark Brown <broonie@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Kent Overstreet
 <kent.overstreet@...ux.dev>, Rick Edgecombe <rick.p.edgecombe@...el.com>,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] sched/membarrier: Fix redundant load of
 membarrier_state

"Nysal Jan K.A." <nysal@...ux.ibm.com> writes:
> On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> is not selected, sync_core_before_usermode() is a no-op.
> In membarrier_mm_sync_core_before_usermode() the compiler does not
> eliminate redundant branches and load of mm->membarrier_state
> for this case as the atomic_read() cannot be optimized away.
>
> Here's a snippet of the code generated for finish_task_switch() on powerpc
> prior to this change:
>
> 1b786c:   ld      r26,2624(r30)   # mm = rq->prev_mm;
> .......
> 1b78c8:   cmpdi   cr7,r26,0
> 1b78cc:   beq     cr7,1b78e4 <finish_task_switch+0xd0>
> 1b78d0:   ld      r9,2312(r13)    # current
> 1b78d4:   ld      r9,1888(r9)     # current->mm
> 1b78d8:   cmpd    cr7,r26,r9
> 1b78dc:   beq     cr7,1b7a70 <finish_task_switch+0x25c>
> 1b78e0:   hwsync
> 1b78e4:   cmplwi  cr7,r27,128
> .......
> 1b7a70:   lwz     r9,176(r26)     # atomic_read(&mm->membarrier_state)
> 1b7a74:   b       1b78e0 <finish_task_switch+0xcc>

Reviewed-by: Michael Ellerman <mpe@...erman.id.au>

cheers