Haren Myneni <ha...@linux.ibm.com> writes:
> VAS allocate, modify and deallocate HCALLs returns
> H_LONG_BUSY_ORDER_1_MSEC or H_LONG_BUSY_ORDER_10_MSEC for busy
> delay and expects OS to reissue HCALL after that delay. But using
> msleep() will often sleep at least 20 msecs even though the
> hypervisor expects to reissue these HCALLs after 1 or 10msecs.
I would word this as "the architecture suggests that the OS reissue
these [...]" instead of framing it as something the platform "expects".
> It might cause these HCALLs takes longer when multiple threads
> issue open or close VAS windows simultaneously.
This is imprecise. Over-sleeping by the OS doesn't cause individual
hcalls to take longer. It is more accurate to say that the higher-level
operation (allocate, modify, free) may take longer than necessary in
cases where the OS must retry the hcalls involved.
> So instead of msleep(), use usleep_range() to ensure sleep with
> the expected value before issuing HCALL again.
>
> Signed-off-by: Haren Myneni <ha...@linux.ibm.com>
> Suggested-by: Nathan Lynch <nath...@linux.ibm.com>
>
> ---
> v1 -> v2:
> - Use usleep_range instead of using RTAS sleep routine as
> suggested by Nathan
> ---
> arch/powerpc/platforms/pseries/vas.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/platforms/pseries/vas.c
> b/arch/powerpc/platforms/pseries/vas.c
> index 71d52a670d95..bade4402741f 100644
> --- a/arch/powerpc/platforms/pseries/vas.c
> +++ b/arch/powerpc/platforms/pseries/vas.c
> @@ -36,9 +36,31 @@ static bool migration_in_progress;
>
> static long hcall_return_busy_check(long rc)
> {
> + unsigned int ms;
This should move down into the H_IS_LONG_BUSY() block if it's not used
outside of it.
> +
> /* Check if we are stalled for some time */
> if (H_IS_LONG_BUSY(rc)) {
> - msleep(get_longbusy_msecs(rc));
> + ms = get_longbusy_msecs(rc);
> + /*
> + * 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 delay should always be 1
> + * or 10msecs, but sleeps 1msec in case if the long
> + * delay is > H_LONG_BUSY_ORDER_10_MSEC.
> + */
> + if (ms > 10)
> + ms = 1;
It's strange to coerce ms to 1 when it's greater than 10. Just clamp it
to 10, e.g.
ms = clamp(get_longbusy_msecs(rc), 1, 10);
> +
> + /*
> + * msleep() will often sleep at least 20 msecs even
> + * though the hypervisor expects to reissue these
> + * HCALLs after 1 or 10msecs. So use usleep_range()
> + * to sleep with the expected value.
> + *
> + * See Documentation/timers/timers-howto.rst on using
> + * the value range in usleep_range().
> + */
> + usleep_range(ms * 100, ms * 1000);
If there's going to be commentary here I think it should just explain
why potentially sleeping for less than the suggested time is OK. There
is wording you can crib in rtas_busy_delay().
> rc = H_BUSY;
> } else if (rc == H_BUSY) {
> cond_resched();
> --
> 2.26.3
