Commit d947fb4c96 ("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
On POWER9 LPARs, the firmwares advertise a very low value of 2us for
CEDE1 exit latency on a Dedicated LPAR. The latency advertized by the
PHYP hypervisor corresponds to the latency required to wakeup from the
underlying hardware idle state. However the wakeup latency from the
LPAR perspective should include
1. The time taken to transition the CPU from the Hypervisor into the
LPAR post wakeup from platform idle state
2. Time taken to send the IPI from the source CPU (waker) to the idle
target CPU (wakee).
1. can be measured via timer idle test, where we queue a timer, say
for 1ms, and enter the CEDE state. When the timer fires, in the timer
handler we compute how much extra timer over the expected 1ms have we
consumed. On a a POWER9 LPAR the numbers are
CEDE latency measured using a timer (numbers in ns)
N Min Median Avg 90%ile 99%ile Max Stddev
400 2601 5677 5668.74 5917 6413 9299 455.01
1. and 2. combined can be determined by an IPI latency test where we
send an IPI to an idle CPU and in the handler compute the time
difference between when the IPI was sent and when the handler ran. We
see the following numbers on POWER9 LPAR.
CEDE latency measured using an IPI (numbers in ns)
N Min Median Avg 90%ile 99%ile Max Stddev
400 711 7564 7369.43 8559 9514 9698 1200.01
Suppose, we consider the 99th percentile latency value measured using
the IPI to be the wakeup latency, the value would be 9.5us This is in
the ballpark of the default value of 10us.
Hence, use the exit latency of CEDE(0) based on the latency values
advertized by platform only from POWER10 onwards. The values
advertized on POWER10 platforms is more realistic and informed by the
latency measurements. For earlier platforms stick to the default value
of 10us. The fix was suggested by Michael Ellerman.
Fixes: d947fb4c96 ("cpuidle: pseries: Fixup exit latency for CEDE(0)")
Reported-by: Enrico Joedecke <joedecke@de.ibm.com>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1626676399-15975-2-git-send-email-ego@linux.vnet.ibm.com
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