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 LzXcNCzN 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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: Ingo Molnar writes: > * Ankur Arora wrote: > >> clear_pages_rep(), clear_pages_erms() use string instructions to zero >> memory. When operating on more than a single page, we can use these >> more effectively by explicitly advertising the region-size to the >> processor, which can use that as a hint to optimize the clearing >> (ex. by eliding cacheline allocation.) >> >> As a secondary benefit, string instructions are typically microcoded, >> and working with larger regions helps amortize the cost of the decode. > > Not just the decoding, but also iterations around page-sized chunks are > not cheap these days: there's various compiler generated mitigations > and other overhead that applies on a typical kernel, and using larger > sizes amortizes that per-page-iteration setup cost. Thanks. Yeah, I was completely forgetting that even the cost of returns has gone up in the mitigation era :D. Is retbleed the one you were alluding to or there might be others that would apply here as well? >> When zeroing the 2MB page, maximize spatial locality by clearing in >> three sections: the faulting page and its immediate neighbourhood, the >> left and the right regions, with the local neighbourhood cleared last. > > s/zeroing the 2MB page > /zeroing a 2MB page > > >> It's not entirely clear why the performance for pg-sz=2MB improves. >> We decode fewer instructions and the hardware prefetcher can do a >> better job, but the perf stats for both of those aren't convincing >> enough to the extent of ~30%. > > s/why the performance > /why performance > >> For both page-sizes, Icelakex, behaves similarly to Milan pg-sz=2MB: we >> see a drop in cycles but there's no drop in cacheline allocation. > > s/Icelakex, behaves similarly > /Icelakex behaves similarly Ack to all of the above. >> Performance for preempt=none|voluntary remains unchanged. > > CONFIG_PREEMPT_VOLUNTARY=y is the default on a number of major > distributions, such as Ubuntu, and a lot of enterprise distro kernels - > and this patch does nothing for them, for no good reason. > So could you please provide a sensible size granularity cutoff of 16MB > or so on non-preemptible kernels, instead of this weird build-time > all-or-nothing binary cutoff based on preemption modes? So, the reason for associating this with preemption modes was in part not the difficulty of deciding a sensible granularity cutoff. I had done a variety of chunking for an earlier version which was a bit of a mess: https://lore.kernel.org/lkml/20220606203725.1313715-11-ankur.a.arora@oracle.com/. Fixed size chunking should be straight-forward enough. However, 16MB is around 1.6ms if you zero at 10GBps. And, longer if you are on older hardware. > On preempt=full/lazy the granularity limit would be infinite. > > I.e the only code dependent on the preemption mode should be the size > cutoff/limit. > On full/lazy preemption the code would, ideally, compile to something > close to your current code. Yeah, agree. >> +obj-$(CONFIG_PREEMPTION) += memory.o > >> +#ifndef CONFIG_HIGHMEM >> +/* >> + * folio_zero_user_preemptible(): multi-page clearing variant of folio_zero_user(). > > We don't care much about HIGHMEM these days I suppose, but this > dependency still feels wrong. Is this a stealth dependency on x86-64, > trying to avoid a new arch Kconfig for this new API, right? ;-) Alas nothing so crafty :). HIGHMEM means that we need to map pages in a hugepage folio via kmap_local_page() -- so cannot treat a hugepage folio as continguous memory and thus cannot use REP; STOS on it. I guess the CONFIG_HIGHMEM condition clearly warrants a comment. -- ankur