Re: [Xen-devel] [PATCH v2] x86/hpet: Improve handling of timer_deadline

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/08/17 14:46, Jan Beulich wrote:
>>>> On 15.08.17 at 15:13, <andrew.cooper3@xxxxxxxxxx> wrote:
>> timer_deadline is only ever updated via this_cpu() in timer_softirq_action(),
>> so is not going to change behind the back of the currently running cpu.
>>
>> Update hpet_broadcast_{enter,exit}() to cache the value in a local variable 
>> to
>> avoid the repeated RELOC_HIDE() penalty.
>>
>> handle_hpet_broadcast() reads the timer_deadlines of remote cpus, but there 
>> is
>> no need to force the read for cpus which are not present in the mask.  One
>> requirement is that we only sample the value once (which happens as a side
>> effect of RELOC_HIDE()), but is made more explicit with ACCESS_ONCE().
>>
>> Bloat-o-meter shows a modest improvement:
>>
>>   add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-144 (-144)
>>   function                                     old     new   delta
>>   hpet_broadcast_exit                          335     313     -22
>>   hpet_broadcast_enter                         327     278     -49
>>   handle_hpet_broadcast                        572     499     -73
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> with one nit:
>
>> @@ -714,9 +714,12 @@ void hpet_broadcast_enter(void)
>>      cpumask_set_cpu(cpu, ch->cpumask);
>>  
>>      spin_lock(&ch->lock);
>> -    /* reprogram if current cpu expire time is nearer */
>> -    if ( per_cpu(timer_deadline, cpu) < ch->next_event )
>> -        reprogram_hpet_evt_channel(ch, per_cpu(timer_deadline, cpu), NOW(), 
>> 1);
>> +    /*
>> +     * reprogram if current cpu expire time is nearer.  deadline is never
>> +     * written by a remote cpu, so the value read earlier is still valid.
>> +     */
> Comments should start with an upper case letter.

Ah yes - will fix.

~Andrew

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