linux-kernel - Re: [PATCH 3.19-rc6 v16 1/6] irqchip: gic: Optimize locking in gic_raise

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Date:	Thu, 26 Feb 2015 16:33:55 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Daniel Thompson <daniel.thompson@...aro.org>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	John Stultz <john.stultz@...aro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>
Subject: Re: [PATCH 3.19-rc6 v16 1/6] irqchip: gic: Optimize locking in
 gic_raise_softirq

On Thu, 26 Feb 2015, Daniel Thompson wrote:

> On Thu, 2015-02-26 at 15:31 -0500, Nicolas Pitre wrote:
> > On Tue, 3 Feb 2015, Daniel Thompson wrote:
> > 
> > > Currently gic_raise_softirq() is locked using upon irq_controller_lock.
> > > This lock is primarily used to make register read-modify-write sequences
> > > atomic but gic_raise_softirq() uses it instead to ensure that the
> > > big.LITTLE migration logic can figure out when it is safe to migrate
> > > interrupts between physical cores.
> > > 
> > > This is sub-optimal in closely related ways:
> > > 
> > > 1. No locking at all is required on systems where the b.L switcher is
> > >    not configured.
> > 
> > ACK
> > 
> > > 2. Finer grain locking can be used on systems where the b.L switcher is
> > >    present.
> > 
> > NAK
> > 
> > Consider this sequence:
> > 
> > 	CPU 1				CPU 2
> > 	-----				-----
> > 	gic_raise_softirq()		gic_migrate_target()
> > 	bl_migration_lock() [OK]
> > 	[...]				[...]
> > 	map |= gic_cpu_map[cpu];	bl_migration_lock() [contended]
> > 	bl_migration_unlock(flags);	bl_migration_lock() [OK]
> > 					gic_cpu_map[cpu] = 1 << new_cpu_id;
> > 					bl_migration_unlock(flags);
> > 					[...]
> > 					(migrate pending IPI from old CPU)
> > 	writel_relaxed(map to GIC_DIST_SOFTINT);
> 
> Isn't this solved inside gic_raise_softirq? How can the writel_relaxed()
> escape from the critical section and happen at the end of the sequence?

Hmmm... blah.  OK I obviously can't read today.

The patch is fine of course.


> > Oh, and a minor nit:
> > 
> > > + * This lock is used by the big.LITTLE migration code to ensure no IPIs
> > > + * can be pended on the old core after the map has been updated.
> > > + */
> > > +#ifdef CONFIG_BL_SWITCHER
> > > +static DEFINE_RAW_SPINLOCK(cpu_map_migration_lock);
> > > +
> > > +static inline void bl_migration_lock(unsigned long *flags)
> > 
> > Please name it gic_migration_lock. "bl_migration_lock" is a bit too 
> > generic in this context.
> 
> I'll change this.

Good.  You may add my ACK.


Nicolas
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