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? ... > 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). > 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? > 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). Thanks, Vlastimil
