On Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote: > On Thu, 21 Oct 2021, Uladzislau Rezki wrote: > > > On Thu 21-10-21 21:13:35, Neil Brown wrote: > > > > On Thu, 21 Oct 2021, Uladzislau Rezki wrote: > > > > > On Wed, Oct 20, 2021 at 05:00:28PM +0200, Uladzislau Rezki wrote: > > > > > > > > > > > > > > On Wed 20-10-21 16:29:14, Uladzislau Rezki wrote: > > > > > > > > On Wed, Oct 20, 2021 at 4:06 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > [...] > > > > > > > > > As I've said I am OK with either of the two. Do you or anybody have any > > > > > > > > > preference? Without any explicit event to wake up for neither of the two > > > > > > > > > is more than just an optimistic retry. > > > > > > > > > > > > > > > > > From power perspective it is better to have a delay, so i tend to say > > > > > > > > that delay is better. > > > > > > > > > > > > > > I am a terrible random number generator. Can you give me a number > > > > > > > please? > > > > > > > > > > > > > Well, we can start from one jiffy so it is one timer tick: schedule_timeout(1) > > > > > > > > > > > A small nit, it is better to replace it by the simple msleep() call: msleep(jiffies_to_msecs(1)); > > > > > > > > I disagree. I think schedule_timeout_uninterruptible(1) is the best > > > > wait to sleep for 1 ticl > > > > > > > > msleep() contains > > > > timeout = msecs_to_jiffies(msecs) + 1; > > > > and both jiffies_to_msecs and msecs_to_jiffies might round up too. > > > > So you will sleep for at least twice as long as you asked for, possible > > > > more. > > > > > > That was my thinking as well. Not to mention jiffies_to_msecs just to do > > > msecs_to_jiffies right after which seems like a pointless wasting of > > > cpu cycle. But maybe I was missing some other reasons why msleep would > > > be superior. > > > > > > > To me the msleep is just more simpler from semantic point of view, i.e. > > it is as straight forward as it can be. In case of interruptable/uninteraptable > > sleep it can be more confusing for people. > > I agree that msleep() is more simple. I think adding the > jiffies_to_msec() substantially reduces that simplicity. > > > > > When it comes to rounding and possibility to sleep more than 1 tick, it > > really does not matter here, we do not need to guarantee exact sleeping > > time. > > > > Therefore i proposed to switch to the msleep(). > > If, as you say, the precision doesn't matter that much, then maybe > msleep(0) > which would sleep to the start of the next jiffy. Does that look a bit > weird? If so, the msleep(1) would be ok. > Agree, msleep(1) looks much better rather then converting 1 jiffy to milliseconds. Result should be the same. > However now that I've thought about some more, I'd much prefer we > introduce something like > memalloc_retry_wait(); > > and use that everywhere that a memory allocation is retried. > I'm not convinced that we need to wait at all - at least, not when > __GFP_DIRECT_RECLAIM is used, as in that case alloc_page will either > - succeed > - make some progress a reclaiming or > - sleep > > However I'm not 100% certain, and the behaviour might change in the > future. So having one place (the definition of memalloc_retry_wait()) > where we can change the sleeping behaviour if the alloc_page behavour > changes, would be ideal. Maybe memalloc_retry_wait() could take a > gfpflags arg. > At sleeping is required for __get_vm_area_node() because in case of lack of vmap space it will end up in tight loop without sleeping what is really bad. Thanks! -- Vlad Rezki
