linux-kernel - [PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

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Message-Id: <1599125247-28488-1-git-send-email-ego@linux.vnet.ibm.com>
Date:   Thu,  3 Sep 2020 14:57:27 +0530
From:   "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>
Cc:     linux-pm@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Subject: [PATCH v2] cpuidle-pseries: Fix CEDE latency conversion from tb to us

From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>

commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
CEDE(0)") sets the exit latency of CEDE(0) based on the latency values
of the Extended CEDE states advertised by the platform. The values
advertised by the platform are in timebase ticks. However the cpuidle
framework requires the latency values in microseconds.

If the tb-ticks value advertised by the platform correspond to a value
smaller than 1us, during the conversion from tb-ticks to microseconds,
in the current code, the result becomes zero. This is incorrect as it
puts a CEDE state on par with the snooze state.

This patch fixes this by rounding up the result obtained while
converting the latency value from tb-ticks to microseconds. It also
prints a warning in case we discover an extended-cede state with
wakeup latency to be 0. In such a case, ensure that CEDE(0) has a
non-zero wakeup latency.

Fixes: commit d947fb4c965c ("cpuidle: pseries: Fixup exit latency for
CEDE(0)")

Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
---
v1-->v2: Added a warning if a CEDE state has 0 wakeup latency (Suggested by Joel Stanley)
         Also added code to ensure that CEDE(0) has a non-zero wakeup latency.	 
 drivers/cpuidle/cpuidle-pseries.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index ff6d99e..a2b5c6f 100644
--- a/drivers/cpuidle/cpuidle-pseries.c
+++ b/drivers/cpuidle/cpuidle-pseries.c
@@ -361,7 +361,10 @@ static void __init fixup_cede0_latency(void)
 	for (i = 0; i < nr_xcede_records; i++) {
 		struct xcede_latency_record *record = &payload->records[i];
 		u64 latency_tb = be64_to_cpu(record->latency_ticks);
-		u64 latency_us = tb_to_ns(latency_tb) / NSEC_PER_USEC;
+		u64 latency_us = DIV_ROUND_UP_ULL(tb_to_ns(latency_tb), NSEC_PER_USEC);
+
+		if (latency_us == 0)
+			pr_warn("cpuidle: xcede record %d has an unrealistic latency of 0us.\n", i);
 
 		if (latency_us < min_latency_us)
 			min_latency_us = latency_us;
@@ -378,10 +381,14 @@ static void __init fixup_cede0_latency(void)
 	 * Perform the fix-up.
 	 */
 	if (min_latency_us < dedicated_states[1].exit_latency) {
-		u64 cede0_latency = min_latency_us - 1;
+		/*
+		 * We set a minimum of 1us wakeup latency for cede0 to
+		 * distinguish it from snooze
+		 */
+		u64 cede0_latency = 1;
 
-		if (cede0_latency <= 0)
-			cede0_latency = min_latency_us;
+		if (min_latency_us > cede0_latency)
+			cede0_latency = min_latency_us - 1;
 
 		dedicated_states[1].exit_latency = cede0_latency;
 		dedicated_states[1].target_residency = 10 * (cede0_latency);
-- 
1.9.4