[PATCH v3] powerpc/pseries/vas: Use usleep_range() to support HCALL delay

[Haren Myneni]

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 suggests OS reissue these HCALLs after 1 or 10msecs.
The open and close VAS window functions hold mutex and then issue
these HCALLs. So these operations can take longer than the
necessary when multiple threads issue open or close window APIs
simultaneously.

So instead of msleep(), use usleep_range() to ensure sleep with
the expected value before issuing HCALL again.

Signed-off-by: Haren Myneni <haren at linux.ibm.com>
Suggested-by: Nathan Lynch <nathanl at linux.ibm.com>

---
v1 -> v2:
- Use usleep_range instead of using RTAS sleep routine as
  suggested by Nathan
v2 -> v3:
- Sleep 10MSecs even for HCALL delay > 10MSecs and the other
  commit / comemnt changes as suggested by Nathan and Ellerman.
---
 arch/powerpc/platforms/pseries/vas.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

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);
 		rc = H_BUSY;
 	} else if (rc == H_BUSY) {
 		cond_resched();
-- 
2.26.3