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From: Leonardo Bras <leonardo@linux.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Enrico Weigelt <info@metux.net>,
	Allison Randal <allison@lohutok.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash
Date: Mon, 30 Mar 2020 11:12:45 -0300
Message-ID: <d52b7462dfa5189a9e7590d6db88bc22b8c2ac5d.camel@linux.ibm.com> (raw)
In-Reply-To: <20200330110231.GG20696@hirez.programming.kicks-ass.net>


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Hello Peter, 

On Mon, 2020-03-30 at 13:02 +0200, Peter Zijlstra wrote:
>  		do {
> > > +			if (unlikely(crash_skip_spinlock))
> > > +				return;
> > 
> > You are adding a test that reads a global var in the middle of a so hot path
> > ? That must kill performance. Can we do different ?
> 
> This; adding code to a super hot patch like this for an exceptional case
> like the crash handling seems like a very very bad trade to me.
> 
> One possible solution is to simply write 0 to the affected spinlocks
> after sending the NMI IPI thing, no?

Yes, I agree.
I suggested this on a comment in v2 of this patch:
http://patchwork.ozlabs.org/patch/1262468/



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      reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-26 22:28 Leonardo Bras
2020-03-26 23:26 ` Leonardo Bras
2020-03-27  6:50 ` Christophe Leroy
2020-03-27 15:51   ` Leonardo Bras
2020-03-28 10:19     ` Christophe Leroy
2020-03-30 14:33       ` Leonardo Bras
2020-03-30 11:02   ` Peter Zijlstra
2020-03-30 14:12     ` Leonardo Bras [this message]

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