On 4/9/22 10:52, Kirill A. Shutemov wrote: > On Fri, Apr 08, 2022 at 12:11:58PM -0700, Dave Hansen wrote: >> On 4/5/22 16:43, Kirill A. Shutemov wrote: >>> Kernel only needs to accept memory once after boot, so during the boot >>> and warm up phase there will be a lot of memory acceptance. After things >>> are settled down the only price of the feature if couple of checks for >>> PageUnaccepted() in allocate and free paths. The check refers a hot >>> variable (that also encodes PageBuddy()), so it is cheap and not visible >>> on profiles. >> >> Let's also not sugar-coat this. Page acceptance is hideously slow. >> It's agonizingly slow. To boot, it's done holding a global spinlock >> with interrupts disabled (see patch 6/8). At the very, very least, each >> acceptance operation involves a couple of what are effectively ring >> transitions, a 2MB memset(), and a bunch of cache flushing. >> >> The system is going to be downright unusable during this time, right? ... >> Do we need anything more discrete to tell users when acceptance is over? > > I can imagine setups that where acceptance is never over. A VM running > a workload with fixed dataset can have planty of memory unaccepted. > > I don't think "make it over" should be the goal. I agree, there will be users that don't care when acceptance is over. But, I'm also sure that there are users that will care deeply. >> For instance, maybe they run something and it goes really slow, they >> watch "accept_memory" until it stops. They rejoice at their good >> fortune! Then, memory allocation starts falling over to a new node and >> the agony beings anew. >> >> I can think of dealing with this in two ways: >> >> cat /sys/.../unaccepted_pages_left >> >> which just walks the bitmap and counts the amount of pages remaining. or >> something like: >> >> echo 1 > /sys/devices/system/node/node0/make_the_pain_stop >> >> Which will, well, make the pain stop on node0. > > Sure we can add handles. But API is hard. Maybe we should wait and see > what is actually needed. (Yes, I'm lazy.:) Let's just call out the possible (probable?) need for new ABI here. Maybe it will cue folks who care to speak up.
