Subject: Re: [PATCH v2] powerpc/pseries/vas: Use usleep_range() to support HCALL delay

From: Michael Ellerman <[email-masked]>
Date: Thu, 30 Nov 2023 13:07:59 +1100

Haren Myneni <[email-masked]> 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.
> It might cause these HCALLs takes longer when multiple threads
> issue open or close VAS windows simultaneously.
>
> So instead of msleep(), use usleep_range() to ensure sleep with
> the expected value before issuing HCALL again.
>
> Signed-off-by: Haren Myneni <[email-masked]>
> Suggested-by: Nathan Lynch <[email-masked]>
>
> ---
> 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;
> +
>  	/* 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;
 
I don't understand this. The hypervisor asked you to sleep for more than
10 milliseconds, so instead you sleep for 1?

I can understand that we don't want to usleep() for the longer durations
that could be returned, but so shouldn't the code be using msleep() for
those values?

Sleeping for a very short duration definitely seems wrong.

> +		/*
> +		 * msleep() will often sleep at least 20 msecs even
> +		 * though the hypervisor expects to reissue these
 
That makes it sound like the hypervisor is reissuing the hcalls.

Better would be "the hypervisor suggests the kernel should reissue the
hcall after ...".

> +		 * 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 ms == 1, then that's 100 usecs, which is not 1 millisecond?

Please use USEC_PER_MSEC.

>  		rc = H_BUSY;
>  	} else if (rc == H_BUSY) {
>  		cond_resched();

cheers