cpuidle: pseries: Fix CEDE latency conversion from tb to us

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: d947fb4c965c ("cpuidle: pseries: Fixup exit latency for CEDE(0)")
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1599125247-28488-1-git-send-email-ego@linux.vnet.ibm.com

diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
index ff6d99e923a49e44440c598cf3edfc8ec60881f8..a2b5c6f60cf0e568cdd6b13a5fd396c4bba4b879 100644 (file)
--- 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);