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Date: Fri, 24 Jul 2015 19:16:58 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: Chris Metcalf <cmetcalf@...hip.com> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Preeti U Murthy <preeti@...ux.vnet.ibm.com>, Christoph Lameter <cl@...ux.com>, Ingo Molnar <mingo@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org>, Rik van Riel <riel@...hat.com> Subject: Re: [PATCH 05/10] nohz: New tick dependency mask On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote: > On 07/23/2015 12:42 PM, Frederic Weisbecker wrote: > >+unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit, > >+ unsigned long *dep) > >+{ > >+ unsigned long prev; > >+ unsigned long old = *dep; > >+ unsigned long mask = BIT_MASK(bit); > >+ > >+ while ((prev = cmpxchg(dep, old, old | mask)) != old) { > >+ old = prev; > >+ cpu_relax(); > >+ } > >+ > >+ return prev; > >+} > > Why not use set_bit() here? It is suitably atomic. Because I don't want to send an IPI if the CPU already had bits set in the dependency. Ideally I need something like test_and_set_bit() but which returns the whole previous value and not just the previous value of the bit. > > >+ /* > >+ * We need the IPIs to be sent from sane process context. > >+ * The posix cpu timers are always set with irqs disabled. > >+ */ > > The block comment indentation is not quite right here. Right. > > >+void tick_nohz_set_tick_dependency_cpu(enum tick_dependency_bit bit, int cpu) > >+{ > >+ unsigned long prev; > >+ struct tick_sched *ts; > >+ > >+ ts = per_cpu_ptr(&tick_cpu_sched, cpu); > >+ > >+ prev = __tick_nohz_set_tick_dependency(bit, &ts->tick_dependency); > >+ if (!prev) > >+ tick_nohz_full_kick_cpu(cpu); > >+} > > I could imagine arguments for a WARN_ON() if cpu == smp_processor_id() here, > since then you should be using the _thiscpu() variant. > Or, you could transparently call the _thiscpu() variant in that case. > I think some comment explaining why the approach you chose is better > than those alternatives would be helpful here, perhaps. It's fine to use this variant for local dependency but if you want it to be NMI safe you need the _this_cpu() version. perf events require it. -- 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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