kernel/sched/fair.c | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-)
Running on large systems nohz.nr_cpus cacheline was seen as contended. There is atomic inc/dec and read happening on many CPUs at a time and it is possible for this line to bounce often. 1st and 2nd patch are minor ones. Looks like correct things to do. Not very important ones. 3rd patch: Main patch which is to get rid of nr_cpus.Instead, use the cpumask which is always updated alongside with it. Functionally it should serve the same purpose. Rest of the fields aren't updated that often. So this line shouldn't bounce that often. Contention issue with nohz.idle_cpus_mask still remains. Mostly it is in separate cacheline than nohz. There are ongoing efforts to mitigate it. It is not addressed by this series. v3 -> v4: - Added to changelog on one less cacheline being dirtied on idle entry/exit (Valentin Schneider) v2 -> v3: - Converted out to return when there are no CPU is in tickless mode since find_ilb_cpu returns anyway (K Prateek Nayak) v1 -> v2: - Dropped patch to check has_blocked based on time. - Detailed changelog for removing nr_cpus (Thanks to Ingo Molnar) v1: https://lore.kernel.org/all/20251201183146.74443-1-sshegde@linux.ibm.com/ v2: https://lore.kernel.org/all/20260102124744.360872-1-sshegde@linux.ibm.com/ v3: https://lore.kernel.org/all/20260107065125.669668-1-sshegde@linux.ibm.com/ Shrikanth Hegde (3): sched/fair: Move checking for nohz cpus after time check sched/fair: Change likelyhood of nohz.nr_cpus sched/fair: Remove nohz.nr_cpus and use weight of cpumask instead kernel/sched/fair.c | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-) -- 2.47.3
Hello Shrikanth,
On 1/12/2026 10:34 AM, Shrikanth Hegde wrote:
> Running on large systems nohz.nr_cpus cacheline was seen as contended.
> There is atomic inc/dec and read happening on many
> CPUs at a time and it is possible for this line to bounce often.
>
> 1st and 2nd patch are minor ones. Looks like correct things to do.
> Not very important ones.
>
> 3rd patch: Main patch which is to get rid of nr_cpus.Instead, use the cpumask
> which is always updated alongside with it. Functionally it should serve
> the same purpose. Rest of the fields aren't updated that often. So this
> line shouldn't bounce that often.
>
> Contention issue with nohz.idle_cpus_mask still remains. Mostly it is in
> separate cacheline than nohz. There are ongoing efforts to mitigate it. It
> is not addressed by this series.
>
> v3 -> v4:
> - Added to changelog on one less cacheline being dirtied on idle
> entry/exit (Valentin Schneider)
I tested the v3 over the weekend and didn't spot any regressions
(at least none that I can reproduce consistently) so feel free to
include:
Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
If anyone is curious, following are results from my setup
(3rd Generation EPYC, 2 socket x 64/128T, boost on, C2 disabled):
Note: tbench hit some insane luck on higher utilization runs. I
haven't been able to reproduce those regressions reliably.
Most data points that show regression also have high run to run
variance on both tip and tip + patch making them unreliable.
==================================================================
Test : hackbench
Units : Normalized time in seconds
Interpretation: Lower is better
Statistic : AMean
==================================================================
Case: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1-groups 1.00 [ -0.00]( 6.43) 1.04 [ -3.60](15.15)
2-groups 1.00 [ -0.00]( 5.42) 1.02 [ -2.17]( 3.57)
4-groups 1.00 [ -0.00]( 2.72) 0.99 [ 0.84]( 3.11)
8-groups 1.00 [ -0.00]( 3.65) 1.00 [ 0.31]( 2.50)
16-groups 1.00 [ -0.00]( 2.26) 1.02 [ -1.67]( 2.92)
==================================================================
Test : tbench
Units : Normalized throughput
Interpretation: Higher is better
Statistic : AMean
==================================================================
Clients: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1 1.00 [ 0.00]( 0.40) 1.00 [ -0.25]( 1.22)
2 1.00 [ 0.00]( 1.33) 0.99 [ -0.57]( 0.37)
4 1.00 [ 0.00]( 0.27) 1.00 [ 0.07]( 0.89)
8 1.00 [ 0.00]( 0.53) 0.99 [ -0.83]( 0.32)
16 1.00 [ 0.00]( 1.39) 1.00 [ 0.11]( 1.92)
32 1.00 [ 0.00]( 1.85) 0.99 [ -1.44]( 3.08)
64 1.00 [ 0.00]( 1.55) 0.98 [ -2.17]( 2.51)
128 1.00 [ 0.00]( 1.05) 0.94 [ -6.11]( 0.28)
256 1.00 [ 0.00]( 0.68) 0.94 [ -5.58]( 3.77)
512 1.00 [ 0.00]( 0.30) 0.95 [ -4.91]( 0.22)
1024 1.00 [ 0.00]( 0.19) 0.95 [ -4.86]( 0.21)
==================================================================
Test : stream-10
Units : Normalized Bandwidth, MB/s
Interpretation: Higher is better
Statistic : HMean
==================================================================
Test: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
Copy 1.00 [ 0.00]( 8.08) 1.03 [ 2.91]( 4.84)
Scale 1.00 [ 0.00]( 5.43) 1.04 [ 3.56]( 3.32)
Add 1.00 [ 0.00]( 5.96) 1.04 [ 4.10]( 2.96)
Triad 1.00 [ 0.00]( 6.36) 0.99 [ -1.23]( 5.83)
==================================================================
Test : stream-100
Units : Normalized Bandwidth, MB/s
Interpretation: Higher is better
Statistic : HMean
==================================================================
Test: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
Copy 1.00 [ 0.00]( 3.78) 1.03 [ 3.17]( 1.90)
Scale 1.00 [ 0.00]( 4.17) 1.02 [ 1.79]( 0.91)
Add 1.00 [ 0.00]( 1.97) 1.01 [ 0.52]( 1.66)
Triad 1.00 [ 0.00]( 2.28) 0.99 [ -1.49]( 4.44)
==================================================================
Test : schbench
Units : Normalized 99th percentile latency in us
Interpretation: Lower is better
Statistic : Median
==================================================================
#workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1 1.00 [ -0.00](33.02) 1.21 [-20.59]( 8.06)
2 1.00 [ -0.00](14.30) 1.14 [-14.29]( 6.45)
4 1.00 [ -0.00]( 2.22) 0.98 [ 2.22]( 4.55)
8 1.00 [ -0.00]( 4.63) 0.94 [ 5.56]( 1.96)
16 1.00 [ -0.00]( 1.67) 1.07 [ -6.67]( 1.82)
32 1.00 [ -0.00]( 5.58) 0.99 [ 1.04]( 2.11)
64 1.00 [ -0.00]( 6.03) 0.99 [ 0.52]( 5.25)
128 1.00 [ -0.00]( 7.09) 1.00 [ -0.49]( 5.11)
256 1.00 [ -0.00]( 3.14) 0.94 [ 6.06](13.53)
512 1.00 [ -0.00]( 0.86) 0.98 [ 2.23]( 1.53)
==================================================================
Test : new-schbench-requests-per-second
Units : Normalized Requests per second
Interpretation: Higher is better
Statistic : Median
==================================================================
#workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1 1.00 [ 0.00]( 0.14) 1.00 [ 0.00]( 0.52)
2 1.00 [ 0.00]( 0.14) 1.00 [ 0.28]( 0.00)
4 1.00 [ 0.00]( 0.14) 1.00 [ 0.00]( 0.00)
8 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.14)
16 1.00 [ 0.00]( 0.00) 1.00 [ 0.00]( 0.00)
32 1.00 [ 0.00]( 5.05) 0.97 [ -3.11]( 1.91)
64 1.00 [ 0.00](10.41) 1.06 [ 5.60]( 3.79)
128 1.00 [ 0.00]( 0.30) 0.98 [ -2.38]( 0.31)
256 1.00 [ 0.00]( 1.43) 0.98 [ -1.73]( 1.38)
512 1.00 [ 0.00]( 1.45) 0.97 [ -3.33]( 1.48)
==================================================================
Test : new-schbench-wakeup-latency
Units : Normalized 99th percentile latency in us
Interpretation: Lower is better
Statistic : Median
==================================================================
#workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1 1.00 [ -0.00](24.99) 1.08 [ -8.33](16.90)
2 1.00 [ -0.00]( 0.00) 1.40 [-40.00](18.20)
4 1.00 [ -0.00](12.06) 1.27 [-27.27]( 7.75)
8 1.00 [ -0.00](14.13) 0.90 [ 10.00](23.66)
16 1.00 [ -0.00](15.96) 1.09 [ -9.09]( 7.45)
32 1.00 [ -0.00](12.06) 0.91 [ 9.09](18.23)
64 1.00 [ -0.00](15.78) 1.06 [ -6.25](13.18)
128 1.00 [ -0.00](10.57) 1.03 [ -3.41]( 5.15)
256 1.00 [ -0.00]( 0.32) 1.00 [ -0.00]( 0.21)
512 1.00 [ -0.00]( 0.00) 1.00 [ 0.38]( 0.20)
==================================================================
Test : new-schbench-request-latency
Units : Normalized 99th percentile latency in us
Interpretation: Lower is better
Statistic : Median
==================================================================
#workers: tip[pct imp](CV) nohz_no_nr_cpus[pct imp](CV)
1 1.00 [ -0.00]( 0.00) 1.00 [ -0.27]( 1.79)
2 1.00 [ -0.00]( 0.74) 0.96 [ 4.07]( 1.90)
4 1.00 [ -0.00]( 0.37) 0.96 [ 3.83]( 1.91)
8 1.00 [ -0.00]( 1.02) 1.00 [ -0.28]( 1.52)
16 1.00 [ -0.00]( 1.61) 1.00 [ 0.28]( 1.86)
32 1.00 [ -0.00]( 9.22) 1.04 [ -3.52]( 6.84)
64 1.00 [ -0.00]( 6.39) 1.06 [ -5.96](22.58)
128 1.00 [ -0.00]( 1.08) 1.12 [-12.43]( 4.61)
256 1.00 [ -0.00]( 6.10) 1.01 [ -0.77]( 4.87)
512 1.00 [ -0.00]( 1.41) 1.01 [ -1.03]( 1.27)
--
Thanks and Regards,
Prateek
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