On Tue, Aug 15, 2023 at 09:27:26AM +0800, Huang, Ying wrote: > Byungchul Park <byungchul@xxxxxx> writes: > > > Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'. > > > > We always face the migration overhead at either promotion or demotion, > > while working with tiered memory e.g. CXL memory and found out TLB > > shootdown is a quite big one that is needed to get rid of if possible. > > > > Fortunately, TLB flush can be defered or even skipped if both source and > > destination of folios during migration are kept until all TLB flushes > > required will have been done, of course, only if the target PTE entries > > have read only permission, more precisely speaking, don't have write > > permission. Otherwise, no doubt the folio might get messed up. > > > > To achieve that: > > > > 1. For the folios that have only non-writable TLB entries, prevent > > TLB flush by keeping both source and destination of folios during > > migration, which will be handled later at a better time. > > > > 2. When any non-writable TLB entry changes to writable e.g. through > > fault handler, give up CONFIG_MIGRC mechanism so as to perform > > TLB flush required right away. > > > > 3. TLB flushes can be skipped if all TLB flushes required to free the > > duplicated folios have been done by any reason, which doesn't have > > to be done from migrations. > > > > 4. Adjust watermark check routine, __zone_watermark_ok(), with the > > number of duplicated folios because those folios can be freed > > and obtained right away through appropreate TLB flushes. > > > > 5. Perform TLB flushes and free the duplicated folios pending the > > flushes if page allocation routine is in trouble due to memory > > pressure, even more aggresively for high order allocation. > > Is the optimization restricted for page migration only? Can it be used > for other places? Like page reclaiming? I don't get you but if I'm missing something here and your idea is something good, then it can be a future work. Byungchul
