Re: [PATCH v2] context_tracking: remove local_irq_save from __acct_update_integrals

From: Rik van Riel
Date: Tue Apr 28 2015 - 08:53:57 EST

On 04/27/2015 07:18 AM, Heiko Carstens wrote:
> On Sat, Apr 25, 2015 at 08:50:49AM -0400, Rik van Riel wrote:
>> On 04/25/2015 05:43 AM, Heiko Carstens wrote:
>>> ...the READ_ONCE() doesn't give you any guarantees about reading
>>> tsk->acct_timexpd in an atomic way.
>>> Well, actually you don't need atomic semantics, but only to make sure that
>>> the read access happens with a single instruction, since you want to protect
>>> against interrupts.
>>> But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result
>>> in two instructions on 32 bit architectures.
>>> (or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)
>> Even if there is (maybe some ARM system?), can we even guarantee
>> that a single instruction to read 64 bits exists on such a system?
> I wouldn't bet on it. I can only talk for s390 and there is an instruction
> available which would do that. But since s390 is now a 64 bit only architecture
> it doesn't matter anyway.
> For other architectures I'd say: no, you can't rely on that.

So what can I do to move forward with this patch?

It speeds up syscall entry / exit by 7% when nohz_full
is enabled on a CPU...

Should I have the irq block compiled in only when
sizeof(cputime_t) > sizeof(long) ?

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