On Tue, Aug 04, 2020 at 07:02:14PM +0200, Vlastimil Babka wrote: > On 8/3/20 6:30 PM, Uladzislau Rezki (Sony) wrote: > > Some background and kfree_rcu() > > =============================== > > The pointers to be freed are stored in the per-cpu array to improve > > performance, to enable an easier-to-use API, to accommodate vmalloc > > memmory and to support a single argument of the kfree_rcu() when only > > a pointer is passed. More details are below. > > > > In order to maintain such per-CPU arrays there is a need in dynamic > > allocation when a current array is fully populated and a new block is > > required. See below the example: > > > > 0 1 2 3 0 1 2 3 > > |p|p|p|p| -> |p|p|p|p| -> NULL > > > > there are two pointer-blocks, each one can store 4 addresses > > which will be freed after a grace period is passed. In reality > > we store PAGE_SIZE / sizeof(void *). > > So what do you actually have without the dynamic allocation, 8 addresses or > PAGE_SIZE / sizeof(void *) addresses? And how many dynamically allocated pages > did you observe you might need in practice? Can it be somehow quantified the > benefit that you are able to allocate up to X pages dynamically from the > pcplists, vs a fixed number of pages held just for that purpose + fallback? > We have PAGE_SIZE / sizeof(void *). The above ASCI was an example :) Answering the second question about fixed number of preloaded pages. Please see some concerns: - It is hard to achieve because the logic does not stick to certain static test case, i.e. it depends on how heavily kfree_rcu(single/double) are used. Based on that, "how heavily" - number of pages are formed, until the drain/reclaimer thread frees them. - Preloading pages and keeping them for internal use, IMHO, seems not optimal from the point of resources wasting. It is better to have a fast mechanism to request a page and release it back for needs of others. As described about we do not know how much we will need. - As for fallback. That is something we would like to avoid(please see the cover letter). Just mention here one concern. For single argument it an entrance to synchronize_rcu() that can significantly slow down the reclamation process. What actually we would like to speed up. > > > A number of pre-fetched elements seems does not depend on amount of the > > physical memory in a system. In my case it is 63 pages. This step is not > > It may depend, if you tune vm.percpu_pagelist_fraction sysctl. But I wouldn't > know the exact formulas immediately. See pageset_set_high_and_batch(). In any > case for your purpose the 'high' value (in e.g. /proc/zoneinfo) is more relevant > (it means the maximum pages you might find cached) for you than the 'batch' (how > much is cached in one refill). > Thanks. I will have a look at it :) it is good that we can control it! > > lock-less. It uses spinlock_t for accessing to the body's zone. This > > step is fully covered in the rmqueue_bulk() function. > > > > Summarizing. The __GFP_FAST_TRY covers only [1] and can not do step [2], > > due to the fact that [2] acquires spinlock_t. It implies that it is super > > fast, but a higher rate of fails is also expected. > > > > Usage: __get_free_page(__GFP_FAST_TRY); > > > > 2) There was a proposal from Matthew Wilcox: https://lkml.org/lkml/2020/7/31/1015 > > > > <snip> > > On non-RT, we could make that lock a raw spinlock. On RT, we could > > decline to take the lock. We'd need to abstract the spin_lock() away > > behind zone_lock(zone), but that should be OK. > > <snip> > > > > It would be great to use any existing flag, say GFP_NOWAIT. Suppose we > > decline to take the lock across the page allocator for RT. But there is > > at least one path that does it outside of the page allocator. GFP_NOWAIT > > can wakeup the kswapd, whereas a "wake-up path" uses sleepable lock: > > > > wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait). > > > > Probably it can be fixed by the excluding of waking of the kswapd process > > defining something like below: > > Is something missing here? > I was talking about: how to bypass waking up of the kswapd that uses sleepable lock. So, __get_free_page(0) will give a trick. But of course that is not enough. Because we have prefeatchin pcpl-logic also. > > what is equal to zero and i am not sure if __get_free_page(0) handles > > all that correctly, though it allocates and seems working on my test > > machine! Please note it is related to "if we can reuse existing flags". > > > > In the meantime, please see below for a patch that adds a __GFP_FAST_TRY, > > which can at least serve as a baseline against which other proposals can > > be compared. The patch is based on the 5.8.0-rc3. > > > > Please RFC. > > At first glance __GFP_FAST_TRY (more descriptive name? __GFP_NO_LOCKS?) seems > better than doing weird things with GFP_NOWAIT, but depends on the real benefits > (hence my first questions). > No, i do not want to break GFP_NOWAIT, as Matthew mentioned later :) __GFP_NO_LOCKS looks nice. I think, something like "TRY" should be added as well. For example __GFP_NO_LOCKS_FAST_TRY. I am glad for the reaction on it :) Thank you, Vlastimil! -- Vlad Rezki
