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 j7RVkNQs foO0TSttivdGkJzCixRek6PIt7r9T6jYJ0QZDa5gUwd9AZWjruR0isyhv3MPwNJKy+ohv10eHeVNQrm7dra2OthVqoc+uGpm/B3ef 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: Borislav Petkov writes: > On Tue, Oct 28, 2025 at 11:51:39AM -0700, Ankur Arora wrote: >> The intent was to use a large enough value that enables uarchs which do >> 'REP; STOS' optimizations, but not too large so we end up with high >> preemption latency. > > How is selecting that number tied to uarches which can do REP; STOSB? I assume > you mean REP; STOSB where microcode magic glue aggregates larger moves than > just u64 chunks but only under certain conditions and so on..., and not > REP_GOOD where the microcode doesn't have problems with REP prefixes... Yes, to what you say below. >> > Why isn't this thing determined dynamically during boot or so, instead of >> > hardcoding it this way and then having to change it again later when bandwidth >> > increases? >> >> I thought of doing that but given that the precise value doesn't matter >> very much (and there's enough slack in it in either direction) it seemed >> unnecessary to do at this point. >> >> Also, I'm not sure that a boot determined value would really help given >> that the 'REP; STOS' bandwidth could be high or low based on how >> saturated the bus is. >> >> Clearly some of this detail should have been in my commit message. > > So you want to have, say, 8MB of contiguous range - if possible - and let the > CPU do larger clears. And it depends on the scheduling model. And it depends > on what the CPU can do wrt length aggregation. Close? Yeah pretty much that. Just to restate: - be large enough so CPUs that can optimize, are able to optimize - even in the bad cases (CPUs that don't optimize and/or are generally slow at this optimization): should be fast enough that we have reasonable preemption latency (which is an issue only for voluntary preemption etc) > Well, I would like, please, for this to be properly documented why it was > selected this way and what *all* the aspects were to select it this way so > that we can know why it is there and we can change it in the future if > needed. > > It is very hard to do so if the reasoning behind it has disappeared in the > bowels of lkml... Ack. Yeah I should have documented this way better. Thanks -- ankur