Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue

From: Jan Kiszka
Date: Fri Apr 24 2015 - 05:00:46 EST


On 2015-04-24 08:54, Mike Galbraith wrote:
> On Thu, 2015-04-23 at 17:00 -0400, Steven Rostedt wrote:
>> On Thu, 23 Apr 2015 09:19:26 +0200
>> Mike Galbraith <umgwanakikbuti@xxxxxxxxx> wrote:
>>
>>>> CC kernel/irq_work.o
>>>> In file included from ../include/asm-generic/percpu.h:6:0,
>>>> from ../arch/x86/include/asm/percpu.h:522,
>>>> from ../arch/x86/include/asm/current.h:5,
>>>> from ../arch/x86/include/asm/processor.h:15,
>>>> from ../arch/x86/include/asm/irq_work.h:4,
>>>> from ../include/linux/irq_work.h:47,
>>>> from ../kernel/irq_work.c:11:
>>>> ../kernel/irq_work.c: In function âirq_work_queue_onâ:
>>>> ../kernel/irq_work.c:85:17: error: âhirq_work_listâ undeclared
>>>> (first use in this function)
>>>> &per_cpu(hirq_work_list, cpu));
>>>
>>> Aw poo, so that's just what I _thought_ it was for.
>>
>> It helps optimization but does nothing for undefined symbols.
>>
>> That said, why don't we clean up that irq_work code and at least
>> declare both lists, and get rid of all the #ifdefs. I wonder if gcc is
>> smart enough to not allocate a static variable if it happens to be
>> optimized out?
>
> Nope, it didn't notice a thing.
>
> This is a stab at that cleanup. Usable as is with Jan's ok, or as
> fodder for your bitmaster-9000 patch shredder, or whatever. Box works
> and it makes line count shrink...
>
> I downgraded evolution v3.16->v3.12 to restore its ability to read it's
> own fscking "Preformatted" switch, so whitespace should be fine.
>
> Oh, btw, if anyone (else) makes a 4.1-rt, your rt push work will want
> one of those nifty hirq tags lest box make boom due to trying to do that
> not only way late, but with irqs enabled which pisses sched all off.
>
> Subject: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue
> Date: Thu, 16 Apr 2015 18:28:16 +0200
> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>
> Instead of turning all irq_work requests into lazy ones on -rt, just
> move their execution from hard into soft-irq context.
>
> This resolves deadlocks of ftrace which will queue work from arbitrary
> contexts, including those that have locks held that are needed for
> raising a soft-irq.
>
> Mike: cleanup ifdef mess and kill hirq_work_list. We need two lists,
> and already have them, merely need to select according to work type.
> In -rt all work not tagged for hirq execution is queued to the lazy
> list and runs via irq_work_tick(). Raising SOFTIRQ_TIMER is always
> done via IPI for deadlock safety, if the work item is not a lazy work
> or the tick is stopped, fire IPI immediately, otherwise let it wait.
> IOW, lazy work is lazy in -rt only until someone queues immediate work.

The approach looks good to me, but the commit log deserves a rework now.

Jan

--
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/