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definitions=main-2311080055 X-Proofpoint-GUID: MDxX6szoIXithGUxz0ILiiL3jJmqby_u X-Proofpoint-ORIG-GUID: MDxX6szoIXithGUxz0ILiiL3jJmqby_u X-Rspam-User: X-Stat-Signature: qtwqtu9muy8dc53ungn1m4judd14z8q4 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: C0D55180006 X-HE-Tag: 1699426181-59818 X-HE-Meta: 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 n9fo0t+G 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 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: Steven Rostedt writes: > On Tue, 7 Nov 2023 20:52:39 -0800 (PST) > Christoph Lameter wrote: > >> On Tue, 7 Nov 2023, Ankur Arora wrote: >> >> > This came up in an earlier discussion (See >> > https://lore.kernel.org/lkml/87cyyfxd4k.ffs@tglx/) and Thomas mentioned >> > that preempt_enable/_disable() overhead was relatively minimal. >> > >> > Is your point that always-on preempt_count is far too expensive? >> >> Yes over the years distros have traditionally delivered their kernels by >> default without preemption because of these issues. If the overhead has >> been minimized then that may have changed. Even if so there is still a lot >> of code being generated that has questionable benefit and just >> bloats the kernel. >> >> >> These are needed to avoid adding preempt_enable/disable to a lot of primitives >> >> that are used for synchronization. You cannot remove those without changing a >> >> lot of synchronization primitives to always have to consider being preempted >> >> while operating. >> > >> > I'm afraid I don't understand why you would need to change any >> > synchronization primitives. The code that does preempt_enable/_disable() >> > is compiled out because CONFIG_PREEMPT_NONE/_VOLUNTARY don't define >> > CONFIG_PREEMPT_COUNT. >> >> In the trivial cases it is simple like that. But look f.e. >> in the slub allocator at the #ifdef CONFIG_PREEMPTION section. There is a >> overhead added to be able to allow the cpu to change under us. There are >> likely other examples in the source. >> > > preempt_disable() and preempt_enable() are much lower overhead today than > it use to be. > > If you are worried about changing CPUs, there's also migrate_disable() too. > >> And the whole business of local data >> access via per cpu areas suffers if we cannot rely on two accesses in a >> section being able to see consistent values. >> >> > The intent here is to always have CONFIG_PREEMPT_COUNT=y. >> >> Just for fun? Code is most efficient if it does not have to consider too >> many side conditions like suddenly running on a different processor. This >> introduces needless complexity into the code. It would be better to remove >> PREEMPT_COUNT for good to just rely on voluntary preemption. We could >> probably reduce the complexity of the kernel source significantly. > > That is what caused this thread in the first place. Randomly scattered > "preemption points" does not scale! > > And I'm sorry, we have latency sensitive use cases that require full > preemption. > >> >> I have never noticed a need to preemption at every instruction in the >> kernel (if that would be possible at all... Locks etc prevent that ideal >> scenario frequently). Preemption like that is more like a pipe dream. The intent isn't to preempt at every other instruction in the kernel. As Thomas describes, the idea is that for voluntary preemption kernels resched happens at cond_resched() points which have been distributed heuristically. As a consequence you might get both too little preemption and too much preemption. The intent is to bring preemption in control of the scheduler which can do a better job than randomly placed cond_resched() points. >> High performance kernel solution usually disable >> overhead like that. You are also missing all the ways in which voluntary preemption points are responsible for poor performance. For instance, if you look atq clear_huge_page() it does page by page copy with a cond_resched() call after clearing each page. But if you can expose the full extent to the CPU, it can optimize differently (for the 1GB page it can now elide cacheline allocation): *Milan* mm/clear_huge_page x86/clear_huge_page change (GB/s) (GB/s) pg-sz=2MB 14.55 19.29 +32.5% pg-sz=1GB 19.34 49.60 +156.4% (See https://lore.kernel.org/all/20230830184958.2333078-1-ankur.a.arora@oracle.com/) > Please read the email from Thomas: > > https://lore.kernel.org/lkml/87cyyfxd4k.ffs@tglx/ > > This is not technically getting rid of PREEMPT_NONE. It is adding a new > NEED_RESCHED_LAZY flag, that will have the kernel preempt only when > entering or in user space. It will behave the same as PREEMPT_NONE, but > without the need for all the cond_resched() scattered randomly throughout > the kernel. And a corollary of that is that with a scheduler controlled PREEMPT_NONE a task might end up running to completion where earlier it could have been preempted early because it crossed a cond_resched(). > If the task is in the kernel for more than one tick (1ms at 1000Hz, 4ms at > 250Hz and 10ms at 100Hz), it will then set NEED_RESCHED, and you will > preempt at the next available location (preempt_count == 0). > > But yes, all locations that do not explicitly disable preemption, will now > possibly preempt (due to long running kernel threads). -- ankur