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Date: Mon, 18 Sep 2017 14:46:43 -0400 (EDT) From: Alan Stern <stern@...land.harvard.edu> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> cc: Peter Zijlstra <peterz@...radead.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>, Andrew Hunter <ahh@...gle.com>, Maged Michael <maged.michael@...il.com>, <gromer@...gle.com>, Avi Kivity <avi@...lladb.com>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Dave Watson <davejwatson@...com>, Will Deacon <will.deacon@....com>, Andy Lutomirski <luto@...nel.org>, <linux-arch@...r.kernel.org> Subject: Re: [RFC PATCH 1/2] membarrier: Provide register expedited private command On Mon, 18 Sep 2017, Mathieu Desnoyers wrote: > 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. > > [ Runtime testing on the PowerPC architecture would be welcome. ] > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> > +++ b/arch/powerpc/include/asm/membarrier.h > @@ -0,0 +1,37 @@ > +#ifndef _ASM_POWERPC_MEMBARRIER_H > +#define _ASM_POWERPC_MEMBARRIER_H > + > +static inline void membarrier_arch_sched_in(struct task_struct *prev, > + struct task_struct *next) > +{ > + /* > + * Only need the full barrier when switching between processes. > + */ > + if (likely(!test_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED) > + || prev->mm == next->mm)) > + return; > + > + /* > + * The membarrier system call requires a full memory barrier > + * after storing to rq->curr, before going back to user-space. > + */ > + smp_mb(); > +} > +static inline void membarrier_arch_fork(struct task_struct *t, > + unsigned long clone_flags) > +{ > + /* > + * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread > + * fork is protected by siglock. membarrier_arch_fork is called > + * with siglock held. > + */ > + if (t->mm->membarrier_private_expedited) > + set_ti_thread_flag(t, TIF_MEMBARRIER_PRIVATE_EXPEDITED); > +} Why have two separate bitflags for the same thing? Can't you just use the mm->membarrier_private_expedited flag everywhere and forget about TIF_MEMBARRIER_PRIVATE_EXPEDITED? Alan Stern
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