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