Re: [PATCH v3 0/7] sched: Flatten the pick

[Chen Yu <chen.yu@linux.dev>]

On Tue, Aug 18, 2026 at 11:16:49AM +0200, Peter Zijlstra wrote:
> > > 
> > > Are you using tip:sched/core at commit 68e3748781 ("sched/fair: Fix
> > > flat
> > > hierarchy") for the flat_cg numbers or did you checkout at
> > > 85570f10a4c6
> > > ("sched/eevdf: Move to a single runqueue")?
> > > 
> > > There are a couple fixes for vruntime update and Vincent's
> > > optimizations
> > > for preemption bits which might make a difference to the overall
> > > results.
> > 
> > 
> > Hi Prateek,
> > 
> > thank you, that's a good call. I did checkout at "Move to a single
> > runqueue". Let me try it with the fix included, see how the results are
> > affected.
> 
> I've not yet managed to digest your various benchmark results, but also
> double check that patch 6/7 from this series is not to 'blame' for the
> some of the changes.
> 
> The 0day robot fingered that patch for at least one issue.
> 
> In that case the benchmark threads ended up 'heavier' than before, which
> resulted in less preemptions. Probably ksoftirqd getting ran less and
> causing a regression in network throughput for that thing.
> 

The 0day' regression was triggered by running netperf/netserver using loopback,
so ksoftirqd was not really running very frequently IMO.

> I did suggest trying to change the slice of ksoftirqd down, such that it
> might be ran more readily, but I'm not sure that ever got tried.

The 0day regression was triggered by running netperf/netserver over loopback,
so ksoftirqd was not running very frequently IIUC.

I borrowed a similar machine(no-SMT, every 4 Cores share L2, 192 Cores) as 0day used.
I asked AI to generate a simple test script based on 0day's reproducer
(attached at the end of this email), and successfully reproduced the regression.

TL;DR,
The decision made by wake_affine_weight() seems to be affected by the increased
weight of the wakee under concur mode. As a result, wake_affine_weight() became less
likely to select this_cpu. This reduced preference for this_cpu leads to a higher
L2 miss rate on this platform, and consequently lower throughput.

Test summary:

| Config | Throughput (Mbps) | **L2 miss%** | **LLC miss%** |
| smp + WA_WEIGHT | **470249** | **19.37%** | 60.08% |
| smp + NO_WA_WEIGHT | 305360 | 32.67% | 60.46% |
| concur + WA_WEIGHT | 338770 | 31.45% | 62.85% |
| concur + NO_WA_WEIGHT | 259358 | 36.24% | 61.95% |

We can see if WA_WEIGHT is disabled in smp mode, the performance drops to concur. One
possible reason is that task_h_load(p) returns a very small value in smp mode(just the
issue that concur wants to fix), whereas it returns a "normal" value in concur mode.

In smp mode, the extremly small value returned by task_h_load(p) can be effectively ignored
in the comparison in wake_affine_weight(). As a result, the condition for returning this_cpu
becomes:

cpu_load(cpu_rq(this_cpu)) * 100 < cpu_load(cpu_rq(prev_cpu) * 108

As a result, this_cpu has a higher chance of being selected in smp mode. In concur mode,
task_h_load(p) is on par with cpu_load(), so it cannot be simply ignored and provides a fairer
basis for choosing between this_cpu and prev_cpu. Since netperf/netserver is a sync wakeup, it
prefers to be co-located on the same CPU/L2 domain for better L2 cache locality.

I'm not sure if this is the expected behavior before I further digging into the details,
a hack patch could restore part of the performance, because it increases the ratio to
return this_cpu:

thanks,
Chenyu

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6d881e530f89..8bff8b6698a9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8385,6 +8385,7 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
        unsigned long task_load;
 
        this_eff_load = cpu_load(cpu_rq(this_cpu));
+       task_load = task_h_load(p);
 
        if (sync) {
                unsigned long current_load = task_h_load(current);
@@ -8392,11 +8393,13 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p,
                if (current_load > this_eff_load)
                        return this_cpu;
 
+               /* replace current load with wakee load does no harm */
+               if (sched_feat(WA_SYNC_WEIGHT) && current_load >= task_load)
+                       return this_cpu;
+
                this_eff_load -= current_load;
        }
 
-       task_load = task_h_load(p);
-
        this_eff_load += task_load;
        if (sched_feat(WA_BIAS))
                this_eff_load *= 100;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 8f0dee8fc475..99fabe53fb22 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -129,6 +129,7 @@ SCHED_FEAT(ATTACH_AGE_LOAD, true)
 SCHED_FEAT(WA_IDLE, true)
 SCHED_FEAT(WA_WEIGHT, true)
 SCHED_FEAT(WA_BIAS, true)
+SCHED_FEAT(WA_SYNC_WEIGHT, true)


test command:

NR=$(( $(nproc) * 2 ))
run() {
local mode=$1 waw=$2 tag=$3
echo $mode | sudo -n tee /sys/kernel/debug/sched/cgroup_mode >/dev/null
echo $waw  | sudo -n tee /sys/kernel/debug/sched/features    >/dev/null
sudo -n pkill -x netserver; sleep 1
netserver -4 >/dev/null 2>&1; sleep 1
rm -f /tmp/m4.$tag
for i in $(seq $NR); do
netperf -4 -H 127.0.0.1 -t TCP_MAERTS -l 70 -P 0 >> /tmp/m4.$tag 
2>/dev/null &
done
sleep 15
sudo -n ./tools/perf/perf stat -a \
-e 
l2_request.hit,l2_request.miss,longest_lat_cache.reference,longest_lat_cache.miss 
\
-x, -o /tmp/p4.$tag -- sleep 20
wait
} >/dev/null 2>&1
run smp    WA_WEIGHT    smp_waw
run smp    NO_WA_WEIGHT smp_nowaw
run concur WA_WEIGHT    con_waw
run concur NO_WA_WEIGHT con_nowaw
echo WA_WEIGHT | sudo -n tee /sys/kernel/debug/sched/features >/dev/null


printf "%-12s %10s %8s %10s %10s\n" CONFIG Mbps streams L2miss% LLCmiss%
for t in smp_waw smp_nowaw con_waw con_nowaw; do
   read TP NS < <(awk '{s+=$NF;n++} END{print s, n}' /tmp/m4.$t)
   awk -F, -v t=$t -v tp=$TP -v ns=$NS '
     $3=="l2_request.hit"{h=$1} $3=="l2_request.miss"{m=$1}
     $3=="longest_lat_cache.reference"{r=$1} 
$3=="longest_lat_cache.miss"{lm=$1}
     END{printf "%-12s %10.0f %8d %9.2f%% %9.2f%%\n", t, tp, ns, 
100*m/(h+m), 100*lm/r}' /tmp/p4.$t
done