Messages in this thread Patch in this message | | | From | "陈华才" <> | Subject | Re: [PATCH] MIPS: Change definition of cpu_relax() for Loongson-3 | Date | Sat, 21 Jul 2018 09:35:59 +0800 |
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Hi, Paul,
SFB can improve the memory bandwidth as much as 30%, and we are planning to enable SFB by default. So, we want to control cpu_relax() under CONFIG_CPU_LOONGSON3, not under CONFIG_LOONGSON3_ENHANCEMENT.
Huacai
------------------ Original ------------------ From: "Paul Burton"<paul.burton@mips.com>; Date: Fri, Jul 20, 2018 05:15 AM To: "Huacai Chen"<chenhc@lemote.com>; Cc: "Ralf Baechle"<ralf@linux-mips.org>; "James Hogan"<jhogan@kernel.org>; "linux-mips"<linux-mips@linux-mips.org>; "Fuxin Zhang"<zhangfx@lemote.com>; "wuzhangjin"<wuzhangjin@gmail.com>; "stable"<stable@vger.kernel.org>; "Alan Stern"<stern@rowland.harvard.edu>; "Andrea Parri"<andrea.parri@amarulasolutions.com>; "Will Deacon"<will.deacon@arm.com>; "Peter Zijlstra"<peterz@infradead.org>; "Boqun Feng"<boqun.feng@gmail.com>; "Nicholas Piggin"<npiggin@gmail.com>; "David Howells"<dhowells@redhat.com>; "Jade Alglave"<j.alglave@ucl.ac.uk>; "Luc Maranget"<luc.maranget@inria.fr>; "Paul E. McKenney"<paulmck@linux.vnet.ibm.com>; "Akira Yokosawa"<akiyks@gmail.com>; "LKML"<linux-kernel@vger.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 | |