RE: [PATCH 0/2] arm64: Allow the rescheduling IPI to bypass irq_enter/exit

From: David Laight <David.Laight@ACULAB.COM>
To: 'Marc Zyngier' <maz@kernel.org>,
	LAK <linux-arm-kernel@lists.infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Cc: Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Android Kernel Team <kernel-team@android.com>,
	Valentin Schneider <Valentin.Schneider@arm.com>
Subject: RE: [PATCH 0/2] arm64: Allow the rescheduling IPI to bypass irq_enter/exit
Date: Sun, 1 Nov 2020 14:30:29 +0000	[thread overview]
Message-ID: <7bac58d011f5443b9442eea38b285dfa@AcuMS.aculab.com> (raw)
In-Reply-To: <20201101131430.257038-1-maz@kernel.org>

From: Marc Zyngier
> Sent: 01 November 2020 13:14
> 
> Vincent recently reported [1] that 5.10-rc1 showed a significant
> regression when running "perf bench sched pipe" on arm64, and
> pinpointed it to the recent move to handling IPIs as normal
> interrupts.
> 
> The culprit is the use of irq_enter/irq_exit around the handling of
> the rescheduling IPI, meaning that we enter the scheduler right after
> the handling of the IPI instead of deferring it to the next preemption
> event. This accounts for most of the overhead introduced.
> 
> On architectures that have architected IPIs at the CPU level (x86
> being the obvious one), the absence of irq_enter/exit is natural. ARM
> (both 32 and 64bits) mimicked this behaviour by having some
> arch-specific handling for the interrupts that are used to implement
> IPIs. Moving IPIs on the normal interrupt path introduced the
> regression.
> 
> This couple of patches try to acknowledge the fact that some IPIs are
> "special", in the sense that they don't need to follow the standard
> interrupt handling flow.
> 
> The good news is that it cures the regression on arm64, and could
> be similarly beneficial to both 32bit ARM, MIPS, or any other
> architecture that uses a unique IRQ to represent the scheduler IPI.
> 
> The bad news is that these patches are ugly as sin, and I really don't
> like them. I specially hate that they can give driver authors the idea
> that they can make random interrupts "faster".
> 
> Comments, suggestions and hate mails appreciated, as always.

One problem is that the only explanation is in the cover note.
Nothing in the patches even gives a hint as to what difference
the 'NAKED' flag makes nor why its is applied to one interrupt.

Which all means someone might revert it all as part of a cleanup.
A couple of comments is probably all it needs.

	David

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