linux-kernel - Re: [PATCH] MIPS: Change definition of cpu

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Date:   Thu, 19 Jul 2018 14:15:47 -0700
From:   Paul Burton <paul.burton@...s.com>
To:     Huacai Chen <chenhc@...ote.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
        Fuxin Zhang <zhangfx@...ote.com>,
        Zhangjin Wu <wuzhangjin@...il.com>,
        stable <stable@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Change definition of cpu_relax() for Loongson-3

Hi Huacai,

On Wed, Jul 18, 2018 at 09:15:46AM +0800, Huacai Chen wrote:
> >> diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
> >> index af34afb..a8c4a3a 100644
> >> --- a/arch/mips/include/asm/processor.h
> >> +++ b/arch/mips/include/asm/processor.h
> >> @@ -386,7 +386,17 @@ unsigned long get_wchan(struct task_struct *p);
> >>  #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29])
> >>  #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status)
> >>
> >> +#ifdef CONFIG_CPU_LOONGSON3
> >> +/*
> >> + * Loongson-3's SFB (Store-Fill-Buffer) may get starved when stuck in a read
> >> + * loop. Since spin loops of any kind should have a cpu_relax() in them, force
> >> + * a Store-Fill-Buffer flush from cpu_relax() such that any pending writes will
> >> + * become available as expected.
> >> + */
> >
> > I think "may starve writes" or "may queue writes indefinitely" would be
> > clearer than "may get starved".
>
> Need I change the comment and resend? Or you change the comment and get merged?

I'm happy to fix up the comment - but have a couple more questions.

Looking into the history, would it be fair to say that this is only a
problem after commit 1e820da3c9af ("MIPS: Loongson-3: Introduce
CONFIG_LOONGSON3_ENHANCEMENT") when CONFIG_LOONGSON3_ENHANCEMENT=y,
which adds code to enable the SFB?

If so would it make sense to use CONFIG_LOONGSON3_ENHANCEMENT to select
the use of smp_mb()?

How much does performance gain does enabling the SFB give you? Would it
be reasonable to just disable it, rather than using this workaround?

Thanks,
    Paul