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The RMP scan is the most expensive >> + * part. If a second RMPOPT occurs, it can skip the expensive scan >> + * if they can see a cached result in the reserved processor memory. >> + * >> + * Do RMPOPT on one CPU alone. Then, follow that up with RMPOPT >> + * on every other primary thread. Followers are "designed to" >> + * skip the scan if they see the "cached" scan results. >> + * >> + * Pin the worker to the current CPU for the leader loop so that >> + * this_cpu remains valid and the RMPOPT instruction executes on >> + * the correct CPU. Use migrate_disable() rather than get_cpu() to >> + * prevent migration while still allowing preemption. >> + */ >> + migrate_disable(); >> + this_cpu = smp_processor_id(); >> + >> + cpumask_andnot(follower_mask, rmpopt_cpumask, >> + topology_sibling_cpumask(this_cpu)); >> + >> + for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) { >> + rmpopt(pa); >> + cond_resched(); >> + } >> + migrate_enable(); >> + >> + /* >> + * Followers: run RMPOPT on remaining cores. CPUs cannot go offline >> + * while SNP is active, so the follower set stays valid across the >> + * scan and cpus_read_lock() is uncontended. >> + */ >> + scoped_guard(cpus_read_lock) { > > We can only reach here after disabling hotplug. Do we still to hold the > cpus_read_lock? > Right — hotplug is disabled for the whole SNP-active window and this work only runs while SNP is active, so the follower set is already stable. I'll drop the cpus_read_lock() and document it: /* * Followers: run RMPOPT on the remaining cores. cpus_read_lock() is * intentionally not held here: CPU hotplug is disabled for the entire * time SNP is active (see snp_prepare()), and this work only runs while * SNP is active, so the follower set stays valid across the whole scan. */ I'll also trim the matching sentence in patch 3's commit log ("ensures cpus_read_lock() in the scan is uncontended"). Actually one of the reasons, why i am still holding the cpus_read_lock is that Sashiko is likely to flag the *bare* on_each_cpu_mask() as missing cpus_read_lock() — the comment above captures why it's safe, so the reasoning is in-tree if it comes up. >> + for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) { >> + on_each_cpu_mask(follower_mask, rmpopt_smp, >> + (void *)pa, true); >> + >> + /* Give a chance for other threads to run */ >> + cond_resched(); > > Ever since commit 7dadeaa6e851 ("sched: Further restrict the preemption > modes"), one cannot select PREEMPT_VOLUNTARY on x86 and cond_resched() > is a nop with all the other preemption models (PREEMPT_NONE will not > preempt a kernel thread; PREEMPT_LAZY / PREEMPT / PREEMPT_RT all make > the kernel fully preemptible except for sections with > preempt_count != 0) > > You can drop these cond_resched() calls in your respin. > Will do — with PREEMPT_VOLUNTARY unavailable on x86 and the remaining models fully preemptible, so cond_resched() are nops. Dropping both. Thanks, Ashish >> + } >> + } >> + >> + free_cpumask_var(follower_mask); >> +} >> + >