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Date: Mon, 24 Mar 2014 10:20:29 -0400 From: Steven Rostedt <rostedt@...dmis.org> To: Nicholas Mc Guire <der.herr@...r.at> Cc: linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH RT] drop migrate_disabled section in _cpu_down On Mon, 24 Mar 2014 13:20:25 +0100 Nicholas Mc Guire <der.herr@...r.at> wrote: > HI ! > > > Minor cleanup: drop unnecessary migrate_disable in _cpu_down > > in kernel/cpu.c:_cpu_down > ... > cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu)); > set_cpus_allowed_ptr(current, cpumask); > ... > migrate_disable(); > mycpu = smp_processor_id(); > if (mycpu == cpu) { > printk(KERN_ERR "Yuck! Still on unplug CPU\n!"); > migrate_enable(); > err = -EBUSY; > goto restore_cpus; > } > migrate_enable(); > > If we got migrated off "cpu" then we can not get migrate > back as the cpus_allowed was updated. so if smp_processor_id(); returns > anything != cpu we are fine even if migrated off to a third cpu between > the mycpu = smp_processor_id() and the if (mycpu == cpu). No need for > migration protection here. > > Further the migrate disable is effectively only needed for the condition > check in the if statement - so I think it could be replaced by: > > if (cpu==smp_processor_id()) { > pr_err("Yuck! Still on unplug CPU\n!"); > err = -EBUSY; > goto restore_cpus; > } > > even in the case that the cpus_allowed had not been updated. > > Patch is against 3.12.13-rt21 This is checking for an anomaly, and it's in a very slow path. I feel more comfortable with keeping the code as is, than to change it for a micro optimization. -- Steve > > Signed-off-by: Nicholas Mc Guire <der.herr@...r.at> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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