From: "Gautham R. Shenoy" <e...@linux.vnet.ibm.com> This is the v2 of the fix to change the default behaviour of cede_offline. The previous version can be found here: https://lkml.org/lkml/2019/9/12/222
The main change from v1 is that the patch2 to create a sysfs file to report and control the value of cede_offline_enabled has been dropped. Problem Description: ==================== Currently on Pseries Linux Guests, the offlined CPU can be put to one of the following two states: - Long term processor cede (also called extended cede) - Returned to the Hypervisor via RTAS "stop-self" call. This is controlled by the kernel boot parameter "cede_offline=on/off". By default the offlined CPUs enter extended cede. The PHYP hypervisor considers CPUs in extended cede to be "active" since the CPUs are still under the control fo the Linux Guests. Hence, when we change the SMT modes by offlining the secondary CPUs, the PURR and the RWMR SPRs will continue to count the values for offlined CPUs in extended cede as if they are online. One of the expectations with PURR is that the for an interval of time, the sum of the PURR increments across the online CPUs of a core should equal the number of timebase ticks for that interval. This is currently not the case. In the following data (Generated using https://github.com/gautshen/misc/blob/master/purr_tb.py): SD-PURR = Sum of PURR increments on online CPUs of that core in 1 second SMT=off =========================================== Core SD-PURR SD-PURR (expected) (observed) =========================================== core00 [ 0] 512000000 69883784 core01 [ 8] 512000000 88782536 core02 [ 16] 512000000 94296824 core03 [ 24] 512000000 80951968 SMT=2 =========================================== Core SD-PURR SD-PURR (expected) (observed) =========================================== core00 [ 0,1] 512000000 136147792 core01 [ 8,9] 512000000 128636784 core02 [ 16,17] 512000000 135426488 core03 [ 24,25] 512000000 153027520 SMT=4 =================================================== Core SD-PURR SD-PURR (expected) (observed) =================================================== core00 [ 0,1,2,3] 512000000 258331616 core01 [ 8,9,10,11] 512000000 274220072 core02 [ 16,17,18,19] 512000000 260013736 core03 [ 24,25,26,27] 512000000 260079672 SMT=on =================================================================== Core SD-PURR SD-PURR (expected) (observed) =================================================================== core00 [ 0,1,2,3,4,5,6,7] 512000000 512941248 core01 [ 8,9,10,11,12,13,14,15] 512000000 512936544 core02 [ 16,17,18,19,20,21,22,23] 512000000 512931544 core03 [ 24,25,26,27,28,29,30,31] 512000000 512923800 This patchset addresses this issue by ensuring that by default, the offlined CPUs are returned to the Hypervisor via RTAS "stop-self" call by changing the default value of "cede_offline_enabled" to false. With the patches, we see that the observed value of the sum of the PURR increments across the the online threads of a core in 1-second matches the number of tb-ticks in 1-second. SMT=off =========================================== Core SD-PURR SD-PURR (expected) (observed) =========================================== core00 [ 0] 512000000 512527568 core01 [ 8] 512000000 512556128 core02 [ 16] 512000000 512590016 core03 [ 24] 512000000 512589440 SMT=2 =========================================== Core SD-PURR SD-PURR (expected) (observed) =========================================== core00 [ 0,1] 512000000 512635328 core01 [ 8,9] 512000000 512610416 core02 [ 16,17] 512000000 512639360 core03 [ 24,25] 512000000 512638720 SMT=4 =================================================== Core SD-PURR SD-PURR (expected) (observed) =================================================== core00 [ 0,1,2,3] 512000000 512757328 core01 [ 8,9,10,11] 512000000 512727920 core02 [ 16,17,18,19] 512000000 512754712 core03 [ 24,25,26,27] 512000000 512739040 SMT=on ============================================================== Core SD-PURR SD-PURR (expected) (observed) ============================================================== core00 [ 0,1,2,3,4,5,6,7] 512000000 512920936 core01 [ 8,9,10,11,12,13,14,15] 512000000 512878728 core02 [ 16,17,18,19,20,21,22,23] 512000000 512921192 core03 [ 24,25,26,27,28,29,30,31] 512000000 512924816 Further, the patch gives an improvement of 5% in offlining of a core on POWER8, gives an improvement of 18% in offlining of a core on POWER9, causes a regression of 2.5% in onlining of a core on POWER8, causes a regression of 4.5% in onlining of a core on POWER9. POWER8 ====================================================================== | Operation | Patch status |#Samples|Min |Max |Median|Avg |Stddev| | | | |(ms)|(ms) | (ms) |(ms) | | ====================================================================== | Offline | Without Patch| 20 | 822| 1232| 972 | 986.8 |112.58| | Offline | With Patch | 20 | 831| 1152| 941 | 938.6 | 80.33| | --------- | -------------|--------|----| ----|------|-------|------| | Online | Without Patch| 20 |1460| 1760| 1620 |1591.2 | 82.72| | Online | With Patch | 20 |1489| 1839| 1629 |1629.6 | 94.90| ====================================================================== POWER9 ====================================================================== | Operation | Patch status |#Samples|Min |Max |Median|Avg |Stddev| | | | |(ms)|(ms) | (ms) |(ms) | | ====================================================================== | Offline | Without Patch| 20 |1120|1653 | 1394 |1392.9 |133.63| | Offline | With Patch | 20 | 930|1316 | 1161 |1130.8 |117.76| | --------- | -------------|--------|----| ----|------|-------|------| | Online | Without Patch| 20 |1652|2108 | 1903 |1891.6 |130.74| | Online | With Patch | 20 |1824|2222 | 1960 |1976.1 | 93.98| ====================================================================== Gautham R. Shenoy (1): pseries/hotplug-cpu: Change default behaviour of cede_offline to "off" Documentation/core-api/cpu_hotplug.rst | 2 +- arch/powerpc/platforms/pseries/hotplug-cpu.c | 12 +++++++++++- 2 files changed, 12 insertions(+), 2 deletions(-) -- 1.9.4
