Re: [RFC-PROTOTYPE 1/1] mm: Add __GFP_FAST_TRY flag

[Uladzislau Rezki]

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