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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=x6icFKpwvdMA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=R5sn_bqk9PoPNMjX8L8A:9 X-Proofpoint-GUID: OIRl-3hn6AbEOXnG5t0h5M-c863XNGl6 X-Proofpoint-ORIG-GUID: OIRl-3hn6AbEOXnG5t0h5M-c863XNGl6 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMDI5MDAzOSBTYWx0ZWRfX1/1KMI/aUHWv 7ojCW5XHq8Ya/8CJcAjg2YHicvA3ZoYMVnxTGQ22+QtkHF6p9nnB9juADn0Py5Afbrksxgo/Saq vg2iqR+G43wGnknmm6+/SkHdXWoBa+f1c01uCU1K8gcYX6d0n6uITvf/D0YSpErFY42HNKN2Hhr UfTGjxDol4DlXk4Kv7+fvxr0NZnoEeIqgM4g21v+asMkClo9rNnLx4Jy5rzFyXOSY3qy12B0riG pTxXfxq5EB43Mx4dEyVGd/bEoV8wPq5Bag6gvDHokf7ZXPrJqhf5MKQZH+yFelcekmD6vzPG8eU kC763L00pU+nAw95CKfZYPHxYkJh4HK+4ZJnxzXimvCiVltFZnhu07dNci4GkTrSSzkO6GKYqLE A9mbfP1KbwtA+vEzMbHy3x+4rG2IfQ== 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