Re: [PATCH v7 02/11] task_isolation: add initial support

Chris Metcalf
2015-10-01T19:26:26Z

On 10/01/2015 08:14 AM, Frederic Weisbecker wrote:
> On Mon, Sep 28, 2015 at 11:17:17AM -0400, Chris Metcalf wrote:
>> diff --git a/include/linux/isolation.h b/include/linux/isolation.h
>> new file mode 100644
>> index 000000000000..fd04011b1c1e
>> --- /dev/null
>> +++ b/include/linux/isolation.h
>> @@ -0,0 +1,24 @@
>> +/*
>> + * Task isolation related global functions
>> + */
>> +#ifndef _LINUX_ISOLATION_H
>> +#define _LINUX_ISOLATION_H
>> +
>> +#include <linux/tick.h>
>> +#include <linux/prctl.h>
>> +
>> +#ifdef CONFIG_TASK_ISOLATION
>> +static inline bool task_isolation_enabled(void)
>> +{
>> + return tick_nohz_full_cpu(smp_processor_id()) &&
>> + (current->task_isolation_flags & PR_TASK_ISOLATION_ENABLE);
> Ok, I may be a bit burdening with that but, how about using the regular
> existing task flags, and if needed later we can still introduce a new field
> in struct task_struct?

The problem is still that we have two basic bits ("enabled" and
"strict") plus eight bits of signal number to override SIGKILL.
So we end up with *something* extra in task_struct no matter what.
And, right now it's conveniently the same value as the bits
passed to prctl(), so we don't need to marshall and unmarshall
the prctl() get/set results.

If we could convince ourselves not to do the "settable signal"
stuff I'd agree that use task flags makes sense, but I was
convinced for v2 of the patch series to add a settable signal,
and I suspect it still does make sense.

>> + while (READ_ONCE(dev->next_event.tv64) != KTIME_MAX) {
> You should add a function in tick-sched.c to get the next tick. This
> is supposed to be a private field.

Yes. Or probably better, a function that just says whether the
timer is quiesced. Obviously I'll wait to hear what Thomas says
on this subject first, though.

>> + if (!warned && (jiffies - start) >= (5 * HZ)) {
>> + pr_warn("%s/%d: cpu %d: task_isolation task blocked for %ld seconds\n",
>> + task->comm, task->pid, smp_processor_id(),
>> + (jiffies - start) / HZ);
>> + warned = true;
>> + }
>> + cond_resched();
>> + if (test_thread_flag(TIF_SIGPENDING))
>> + break;
> Why not use signal_pending()?

Makes sense, thanks.

> I still think we could try a wait-wake standard scheme.

I'm curious to hear what you make of my arguments in the
other thread on this subject!

--
Chris Metcalf, EZChip Semiconductor