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peterz@infradead.org, torvalds@linux-foundation.org, 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: <20231109234736.4kik62ys47ey23ju@treble> Message-ID: 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definitions=main-2311100005 X-Proofpoint-GUID: ZQoEwvGcOy18TKHoFt2rE-7c9_DxpQLy X-Proofpoint-ORIG-GUID: ZQoEwvGcOy18TKHoFt2rE-7c9_DxpQLy X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: BFC4740009 X-Stat-Signature: 5psjm35iwxb5haqxifpuuhe8asp93zbb X-HE-Tag: 1699577218-762876 X-HE-Meta: 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 dLezHNlZ Ps0dmf4DBP7GFD+o/1yMqruICLlComigQmOELCPmpzkbcYObaBWXYCwCZL8By5hMbyOH1W4iU9qPBOrmYKh8G6HF9bhoVd1BeIFhXNxQzU5DPMblID5eSNpsuaaLxyX25YSjZNbZWdRDvqCala9DHM3vPTJwes6Mf1w/gXCNCzcWWRGoyYz3KaxgTI8fPbWnNGiZcobhgAadp8m3uj/yrEX4RYlu3xI9QKkVTKPacobGK9t22LMmhHEsv5NlhEcMIEvORxn4GHaH4A35PKTtHZ+dtGoVdT/ByNALq0sNiMBjIHxuFyCnibHkDvKI+HAU3O3gfqyaLTh9wS87MMPKZNGiYnHY40EgrJREE6iYVgE/mF0NccsNqYmCGnkw4mMWnJJ3QqE+lgC/atl09L11W3NUmCZy6hdmH6cJB6h3lggsWYlDOu5j0rZNMWskNH18dVXSZNVxUKgIBK57U58+cTNNqvQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Josh Poimboeuf writes: > On Thu, Nov 09, 2023 at 02:50:48PM -0800, Ankur Arora wrote: >> >> 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. > > Let me back up a bit and explain what klp is trying to do. > > When a livepatch is applied, klp needs to unwind all the tasks, > preferably within a reasonable amount of time. > > We can't unwind task A from task B while task A is running, since task A > could be changing the stack during the unwind. So task A needs to be > blocked or asleep. The only exception to that is if the unwind happens > in the context of task A itself. > The problem we were seeing was CPU-bound kthreads (e.g., vhost_worker) > not getting patched within a reasonable amount of time. We fixed it by > hooking the klp unwind into cond_resched() so it can unwind from the > task itself. Right, so the task calls schedule() itself via cond_resched() and that works. If the task schedules out by calling preempt_enable() that presumably works as well. So, that leaves two paths where we can't unwind: 1. a task never entering or leaving preemptible sections 2. or, a task which gets preempted in irqentry_exit_cond_resched() This we could disable temporarily. > It only worked because we had a non-preempted hook (because non-ORC > unwinders can't unwind reliably through preemption) which called klp to > unwind from the context of the task. > > Without something to hook into, we have a problem. We could of course > hook into schedule(), but if the kthread never calls schedule() from a > non-preempted context then it still doesn't help. Yeah agreed. The first one is a problem. And, that's a problem with the removal of cond_resched() generally. Because the way to fix case 1 was typically to add a cond_resched() when softlockups were seen or in code review. -- ankur