On 25/07/23 06:49, Joel Fernandes wrote: > Interesting series Valentin. Some high-level question/comments on this one: > >> On Jul 20, 2023, at 12:34 PM, Valentin Schneider <vschneid@xxxxxxxxxx> wrote: >> >> text_poke_bp_batch() sends IPIs to all online CPUs to synchronize >> them vs the newly patched instruction. CPUs that are executing in userspace >> do not need this synchronization to happen immediately, and this is >> actually harmful interference for NOHZ_FULL CPUs. > > Does the amount of harm not correspond to practical frequency of text_poke? > How often does instruction patching really happen? If it is very infrequent > then I am not sure if it is that harmful. > Being pushed over a latency threshold *once* is enough to impact the latency evaluation of your given system/application. It's mainly about shielding the isolated, NOHZ_FULL CPUs from whatever the housekeeping CPUs may be up to (flipping static keys, loading kprobes, using ftrace...) - frequency of the interference isn't such a big part of the reasoning. >> >> As the synchronization IPIs are sent using a blocking call, returning from >> text_poke_bp_batch() implies all CPUs will observe the patched >> instruction(s), and this should be preserved even if the IPI is deferred. >> In other words, to safely defer this synchronization, any kernel >> instruction leading to the execution of the deferred instruction >> sync (ct_work_flush()) must *not* be mutable (patchable) at runtime. > > If it is not infrequent, then are you handling the case where userland > spends multiple seconds before entering the kernel, and all this while > the blocking call waits? Perhaps in such situation you want the real IPI > to be sent out instead of the deferred one? > The blocking call only waits for CPUs for which it queued a CSD. Deferred calls do not queue a CSD thus do not impact the waiting at all. See smp_call_function_many_cond().
