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paulmck@kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de, jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, jgross@suse.com, andrew.cooper3@citrix.com, mingo@kernel.org, bristot@kernel.org, mathieu.desnoyers@efficios.com, geert@linux-m68k.org, glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com, mattst88@gmail.com, krypton@ulrich-teichert.org, David.Laight@ACULAB.COM, richard@nod.at, mjguzik@gmail.com, Jiri Kosina , Miroslav Benes , Petr Mladek , Joe Lawrence , live-patching@vger.kernel.org Subject: Re: [RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task switching to cond_resched()" In-reply-to: <20231109175118.olggitpaltz47n3b@treble> Date: Thu, 09 Nov 2023 14:50:48 -0800 Message-ID: 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definitions=main-2311090165 X-Proofpoint-GUID: p5Cny-CtgEwt7uQabnXc63NwiuL_yy0B X-Proofpoint-ORIG-GUID: p5Cny-CtgEwt7uQabnXc63NwiuL_yy0B X-Rspamd-Queue-Id: F3ECB4000C X-Rspam-User: X-Stat-Signature: ugayobpqk9ozbg4bbwp6z54ubrrk8e44 X-Rspamd-Server: rspam01 X-HE-Tag: 1699570282-162502 X-HE-Meta: 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 wmGsSfCz 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This reverts commit e3ff7c609f39671d1aaff4fb4a8594e14f3e03f8. >> > > > >> > > > Note that removing this commit reintroduces "live patches failing to >> > > > complete within a reasonable amount of time due to CPU-bound kthreads." >> > > > >> > > > Unfortunately this fix depends quite critically on PREEMPT_DYNAMIC and >> > > > existence of cond_resched() so this will need an alternate fix. >> > >> > We definitely don't want to introduce a regression, something will need >> > to be figured out before removing cond_resched(). >> > >> > We could hook into preempt_schedule_irq(), but that wouldn't work for >> > non-ORC. >> > >> > Another option would be to hook into schedule(). Then livepatch could >> > set TIF_NEED_RESCHED on remaining unpatched tasks. But again if they go >> > through the preemption path then we have the same problem for non-ORC. >> > >> > Worst case we'll need to sprinkle cond_livepatch() everywhere :-/ >> > >> >> I guess I'm not fully understanding what the cond rescheds are for. But >> would an IPI to all CPUs setting NEED_RESCHED, fix it? Yeah. We could just temporarily toggle to full preemption, when NEED_RESCHED_LAZY is always upgraded to NEED_RESCHED which will then send IPIs. > If all livepatch arches had the ORC unwinder, yes. > > The problem is that frame pointer (and similar) unwinders can't reliably > unwind past an interrupt frame. Ah, I wonder if we could just disable the preempt_schedule_irq() path temporarily? Hooking into schedule() alongside something like this: @@ -379,7 +379,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs) void irqentry_exit_cond_resched(void) { - if (!preempt_count()) { + if (klp_cond_resched_disable() && !preempt_count()) { The problem would be tasks that don't go through any preemptible sections. -- ankur