On Wed, Aug 16, 2023 at 09:01:12AM +0800, Huang, Ying wrote: > Byungchul Park <byungchul@xxxxxx> writes: > > > 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? > > > > Just to make sure, are you talking about the (5) description? For now, > > it's performed at the beginning of __alloc_pages_slowpath(), say, before > > page recaiming. Do you think it'd be meaningful to perform it during page > > reclaiming? Or do you mean something else? > > Not for (5). TLB needs to be flushed during page reclaiming too. Can > similar method be used to reduce TLB flushing there too? If you were talking about unmapping for swap while reclaiming, then I think yes. The case can also take benefit from CONFIG_MIGRC. Byungchul
