On 7/25/23 09:39, Peter Zijlstra wrote: > 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. Ok cool. I guess I still have memories of that old ARM device I had where modifications to kernel text was forbidden by hardware (was a security feature). That was making kprobes unusable... >>> 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. Makes sense. To be fair I received his email 3 minutes before yours ;-). But thank you both for clarifying! - Joel
