14c73bd344
With commit 247f2f6f3c70 ("sched/core: Don't schedule threads on
pre-empted vCPUs"), the scheduler avoids preempted vCPUs to schedule
tasks on wakeup. This leads to wrong choice of CPU, which in-turn
leads to larger wakeup latencies. Eventually, it leads to performance
regression in latency sensitive benchmarks like soltp, schbench etc.
On Powerpc, vcpu_is_preempted() only looks at yield_count. If the
yield_count is odd, the vCPU is assumed to be preempted. However
yield_count is increased whenever the LPAR enters CEDE state (idle).
So any CPU that has entered CEDE state is assumed to be preempted.
Even if vCPU of dedicated LPAR is preempted/donated, it should have
right of first-use since they are supposed to own the vCPU.
On a Power9 System with 32 cores:
# lscpu
Architecture: ppc64le
Byte Order: Little Endian
CPU(s): 128
On-line CPU(s) list: 0-127
Thread(s) per core: 8
Core(s) per socket: 1
Socket(s): 16
NUMA node(s): 2
Model: 2.2 (pvr 004e 0202)
Model name: POWER9 (architected), altivec supported
Hypervisor vendor: pHyp
Virtualization type: para
L1d cache: 32K
L1i cache: 32K
L2 cache: 512K
L3 cache: 10240K
NUMA node0 CPU(s): 0-63
NUMA node1 CPU(s): 64-127
# perf stat -a -r 5 ./schbench
v5.4 v5.4 + patch
Latency percentiles (usec) Latency percentiles (usec)
50.0000th: 45 50.0th: 45
75.0000th: 62 75.0th: 63
90.0000th: 71 90.0th: 74
95.0000th: 77 95.0th: 78
*99.0000th: 91 *99.0th: 82
99.5000th: 707 99.5th: 83
99.9000th: 6920 99.9th: 86
min=0, max=10048 min=0, max=96
Latency percentiles (usec) Latency percentiles (usec)
50.0000th: 45 50.0th: 46
75.0000th: 61 75.0th: 64
90.0000th: 72 90.0th: 75
95.0000th: 79 95.0th: 79
*99.0000th: 691 *99.0th: 83
99.5000th: 3972 99.5th: 85
99.9000th: 8368 99.9th: 91
min=0, max=16606 min=0, max=117
Latency percentiles (usec) Latency percentiles (usec)
50.0000th: 45 50.0th: 46
75.0000th: 61 75.0th: 64
90.0000th: 71 90.0th: 75
95.0000th: 77 95.0th: 79
*99.0000th: 106 *99.0th: 83
99.5000th: 2364 99.5th: 84
99.9000th: 7480 99.9th: 90
min=0, max=10001 min=0, max=95
Latency percentiles (usec) Latency percentiles (usec)
50.0000th: 45 50.0th: 47
75.0000th: 62 75.0th: 65
90.0000th: 72 90.0th: 75
95.0000th: 78 95.0th: 79
*99.0000th: 93 *99.0th: 84
99.5000th: 108 99.5th: 85
99.9000th: 6792 99.9th: 90
min=0, max=17681 min=0, max=117
Latency percentiles (usec) Latency percentiles (usec)
50.0000th: 46 50.0th: 45
75.0000th: 62 75.0th: 64
90.0000th: 73 90.0th: 75
95.0000th: 79 95.0th: 79
*99.0000th: 113 *99.0th: 82
99.5000th: 2724 99.5th: 83
99.9000th: 6184 99.9th: 93
min=0, max=9887 min=0, max=111
Performance counter stats for 'system wide' (5 runs):
context-switches 43,373 ( +- 0.40% ) 44,597 ( +- 0.55% )
cpu-migrations 1,211 ( +- 5.04% ) 220 ( +- 6.23% )
page-faults 15,983 ( +- 5.21% ) 15,360 ( +- 3.38% )
Waiman Long suggested using static_keys.
Fixes: 247f2f6f3c70 ("sched/core: Don't schedule threads on pre-empted vCPUs")
Cc: stable@vger.kernel.org # v4.18+
Reported-by: Parth Shah <parth@linux.ibm.com>
Reported-by: Ihor Pasichnyk <Ihor.Pasichnyk@ibm.com>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Waiman Long <longman@redhat.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
Tested-by: Parth Shah <parth@linux.ibm.com>
[mpe: Move the key and setting of the key to pseries/setup.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191213035036.6913-1-mpe@ellerman.id.au
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.