On Tue, Jul 25, 2023 at 06:49:45AM -0400, 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. Well, it can happen quite a bit, also from things people would not typically 'expect' it. For instance, the moment you create the first per-task perf event we frob some jump-labels (and again some second after the last one goes away). The same for a bunch of runtime network configurations. > > 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? Please re-read what Valentin wrote -- nobody is waiting on anything.
