Messages in this thread | | | Date | Mon, 12 May 2014 01:37:58 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH v2 2/2] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU" |
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On 05/10/2014 08:36 AM, Tejun Heo wrote: > On Wed, May 07, 2014 at 03:31:51AM +0530, Srivatsa S. Bhat wrote: >> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c >> index 01fbae5..7abb361 100644 >> --- a/kernel/stop_machine.c >> +++ b/kernel/stop_machine.c >> @@ -165,12 +165,13 @@ static void ack_state(struct multi_stop_data *msdata) >> set_state(msdata, msdata->state + 1); >> } >> >> + > > Why add a new line here?
Argh, a stray newline.. will remove it.
> >> /* This is the cpu_stop function which stops the CPU. */ >> static int multi_cpu_stop(void *data) >> { >> struct multi_stop_data *msdata = data; >> enum multi_stop_state curstate = MULTI_STOP_NONE; >> - int cpu = smp_processor_id(), err = 0; >> + int cpu = smp_processor_id(), num_active_cpus, err = 0; > > TYPE var0 = INIT0, var1, var2 = INIT2; > > looks kinda weird. Maybe collect initialized ones to one side or > separate out uninitialized one to a separate declaration? >
Yeah, now that you point out, it does look very odd. I don't remember why I wrote it that way in the first place! :-( I'll fix this in the next version. Thanks!
> Also, isn't nr_active_cpus more common way of naming it? >
Sure, will use this convention.
>> unsigned long flags; >> bool is_active; >> >> @@ -180,15 +181,38 @@ static int multi_cpu_stop(void *data) >> */ >> local_save_flags(flags); >> >> - if (!msdata->active_cpus) >> + if (!msdata->active_cpus) { >> is_active = cpu == cpumask_first(cpu_online_mask); >> - else >> + num_active_cpus = 1; >> + } else { >> is_active = cpumask_test_cpu(cpu, msdata->active_cpus); >> + num_active_cpus = cpumask_weight(msdata->active_cpus); >> + } >> >> /* Simple state machine */ >> do { >> /* Chill out and ensure we re-read multi_stop_state. */ >> cpu_relax(); >> + >> + /* >> + * In the case of CPU offline, we don't want the other CPUs to >> + * send IPIs to the active_cpu (the one going offline) after it >> + * has entered the _DISABLE_IRQ state (because, then it will >> + * notice the IPIs only after it goes offline). So ensure that >> + * the active_cpu always follows the others while entering >> + * each subsequent state in this state-machine. >> + * >> + * msdata->thread_ack tracks the number of CPUs that are yet to >> + * move to the next state, during each transition. So make the >> + * active_cpu(s) wait until ->thread_ack indicates that the >> + * active_cpus are the only ones left to complete the transition. >> + */ >> + if (is_active) { >> + /* Wait until all the non-active threads ack the state */ >> + while (atomic_read(&msdata->thread_ack) > num_active_cpus) >> + cpu_relax(); >> + } > > Wouldn't it be cleaner to separate this out to a separate stage so > that there are two separate DISABLE_IRQ stages - sth like > MULTI_STOP_DISABLE_IRQ_INACTIVE and MULTI_STOP_DISABLE_IRQ_ACTIVE? > The above adds an ad-hoc mechanism on top of the existing mechanism > which is built to sequence similar things anyway. >
Indeed, that looks like a much more elegant method! Thanks a lot for the suggestion Tejun, I'll use that in the next version of the patchset.
Thank you!
Regards, Srivatsa S. Bhat
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