On Mon, Jan 25, 2021 at 04:39:43PM +0100, Michal Hocko wrote: > On Mon 25-01-21 15:31:50, Uladzislau Rezki wrote: > > > On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote: > > > > For a single argument we can directly request a page from a caller > > > > context when a "carry page block" is run out of free spots. Instead > > > > of hitting a slow path we can request an extra page by demand and > > > > proceed with a fast path. > > > > > > > > A single-argument kvfree_rcu() must be invoked in sleepable contexts, > > > > and that its fallback is the relatively high latency synchronize_rcu(). > > > > Single-argument kvfree_rcu() therefore uses > > > > GFP_KERNEL|__GFP_RETRY_MAYFAIL > > > > to allow limited sleeping within the memory allocator. > > > > > > __GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy > > > way to allocate without triggering the OOM killer. Is this really what > > > you need/want? Is __GFP_NORETRY too weak? > > > > > Hm... We agreed to proceed with limited lightwait memory direct reclaim. > > Johannes Weiner proposed to go with __GFP_NORETRY flag as a starting > > point: https://www.spinics.net/lists/rcu/msg02856.html > > > > <snip> > > So I'm inclined to suggest __GFP_NORETRY as a starting point, and make > > further decisions based on instrumentation of the success rates of > > these opportunistic allocations. > > <snip> > > I completely agree with Johannes here. > > > but for some reason, i can't find a tail or head of it, we introduced > > __GFP_RETRY_MAYFAIL what is a heavy one from a time consuming point of view. > > What we would like to avoid. > > Not that I object to this use but I think it would be much better to use > it based on actual data. Going along with it right away might become a > future burden to make any changes in this aspect later on due to lack of > exact reasoning. General rule of thumb for __GFP_RETRY_MAYFAIL is really > try as hard as it can get without being really disruptive (like OOM > killing something). And your wording didn't really give me that > impression. > Initially i proposed just to go with GFP_NOWAIT flag. But later on there was a discussion about a fallback path, that uses synchronize_rcu() can be slow, thus minimizing its hitting would be great. So, here we go with a trade off.
Doing it hard as __GFP_RETRY_MAYFAIL can do, is not worth(IMHO), but to have some light-wait requests would be acceptable. That is why __GFP_NORETRY was proposed. There were simple criterias we discussed which we would like to achieve: a) minimize a fallback hitting; b) avoid of OOM involving; c) avoid of dipping into the emergency reserves. See kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument kvfree_rcu() -- Vlad Rezki
