lkml.org 
[lkml]   [2015]   [Feb]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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


\
 
 \ /
  Last update: 2015-02-26 22:41    [W:0.151 / U:0.088 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site