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Reverting the patch resolves this. So I dug >> deeper, found out that things were even worse before [1], but even with >> that, I could still see up to 2 ticks delay per call. If you combine >> that with an unfortunate loop of irq_work_sync calls (mine is >> out-of-tree, but I see something even "worse" in bpf_mem_alloc_destroy), >> there is this impact. >> >> While this patch is motivated by PREEMPT_RT, the condition to enter the >> modified branch are not limited to it: > > It is not motivated by PREEMPT_RT. All the lazy-work has "this" problem. > It is just on RT most of the work is stuffed into the lazy queue. I was suspecting that already, but the patch description still reads rt-only. > >> ... || !arch_irq_work_has_interrupt() >> >> arch_irq_work_has_interrupt() is false on some archs, either always >> (very rare) or under certain conditions. On arm, it's false when >> is_smp() is false. >> >> If we "only" need the synchronize_rcu() for PREEMPT_RT, should we limit >> it to that configuration? Or do we actually need otherwise as well? >> >> And what could be done to accelerate irq_work_sync loops? Practically, a >> single synchronize_rcu() at the end could be enough, no? > > Let me look into this again… > Thanks! I was meanwhile considering to backport the synchronize_rcu accelerator commit. Seems to be straightforward for 6.12, but older kernels need more... Jan -- Siemens AG, Foundational Technologies Linux Expert Center