[PATCH v4] powerpc/pseries/vas: Use usleep_range() to support HCALL delay

[Nathan Lynch]

"Aneesh Kumar K.V" <aneesh.kumar at kernel.org> writes:
> Haren Myneni <haren at linux.ibm.com> writes:
>
>> diff --git a/arch/powerpc/platforms/pseries/vas.c b/arch/powerpc/platforms/pseries/vas.c
>> index 71d52a670d95..5cf81c564d4b 100644
>> --- a/arch/powerpc/platforms/pseries/vas.c
>> +++ b/arch/powerpc/platforms/pseries/vas.c
>> @@ -38,7 +38,30 @@ static long hcall_return_busy_check(long rc)
>>  {
>>  	/* Check if we are stalled for some time */
>>  	if (H_IS_LONG_BUSY(rc)) {
>> -		msleep(get_longbusy_msecs(rc));
>> +		unsigned int ms;
>> +		/*
>> +		 * Allocate, Modify and Deallocate HCALLs returns
>> +		 * H_LONG_BUSY_ORDER_1_MSEC or H_LONG_BUSY_ORDER_10_MSEC
>> +		 * for the long delay. So the sleep time should always
>> +		 * be either 1 or 10msecs, but in case if the HCALL
>> +		 * returns the long delay > 10 msecs, clamp the sleep
>> +		 * time to 10msecs.
>> +		 */
>> +		ms = clamp(get_longbusy_msecs(rc), 1, 10);
>> +
>> +		/*
>> +		 * msleep() will often sleep at least 20 msecs even
>> +		 * though the hypervisor suggests that the OS reissue
>> +		 * HCALLs after 1 or 10msecs. Also the delay hint from
>> +		 * the HCALL is just a suggestion. So OK to pause for
>> +		 * less time than the hinted delay. Use usleep_range()
>> +		 * to ensure we don't sleep much longer than actually
>> +		 * needed.
>> +		 *
>> +		 * See Documentation/timers/timers-howto.rst for
>> +		 * explanation of the range used here.
>> +		 */
>> +		usleep_range(ms * 100, ms * 1000);
>>
>
> Is there more details on this range? (ms *100, ms * 1000)

The preceding comment ("see Documentation/timers/timers-howto...")
should be removed, that document does not really explain this range
directly.

What timers-howto does say is that the larger a range you provide, the
less likely you are to trigger an interrupt to wake up. Since we know
that retrying "too soon" is harmless, providing a lower bound an order
of magnitude less than the suggested delay (which forms the upper bound)
seems reasonable.

>
> can we use USEC_PER_MSEC instead of 1000.

agreed

>>  		rc = H_BUSY;
>>  	} else if (rc == H_BUSY) {
>>  		cond_resched();
>
>
> It would be good to convert this to a helper and switch rtas_busy_delay
> to use this new helper.

I have reservations about that suggestion.

The logic for handling the 990x extended delay constants conceivably
could be shared. But any helper that handles the "retry immediately"
statuses has to know whether it's handling a status from an RTAS call or
an hcall: RTAS_BUSY and H_BUSY have the same semantics but different
values.

Also I don't really want kernel/rtas.c to gain more dependencies on
pseries-specific code as long as there are non-pseries platforms that
use it (chrp, maple, cell).

Tolerating a little duplication here should be OK IMO.

> One question though is w.r.t the clamp values.
> Does that need to be specific to each hcall? Can we make it generic?
>
> rtas_busy_delay() expliclity check for 20msec. Any reason to do that?
> timers-howto.rst suggest > 10msec to use msleep.

I understand it to suggest (roughly) 20ms for the threshold:

        SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
                * Use usleep_range
[...]
                        msleep(1~20) may not do what the caller intends, and
                        will often sleep longer (~20 ms actual sleep for any
                        value given in the 1~20ms range).

20ms is also the threshold used by fsleep().