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[35.238.199.243]) by smtp.gmail.com with ESMTPSA id h2sm9739557ioj.30.2021.03.29.12.28.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Mar 2021 12:28:50 -0700 (PDT) Date: Mon, 29 Mar 2021 19:28:49 +0000 From: Dennis Zhou To: Roman Gushchin Cc: Tejun Heo , Christoph Lameter , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH rfc 1/4] percpu: implement partial chunk depopulation Message-ID: References: <20210324190626.564297-1-guro@fb.com> <20210324190626.564297-2-guro@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 2291940002D0 X-Stat-Signature: 158bi1wgfb6mdi4okpmdbajphb7tbxnn Received-SPF: none (gmail.com>: No applicable sender policy available) receiver=imf26; identity=mailfrom; envelope-from=""; helo=mail-io1-f50.google.com; client-ip=209.85.166.50 X-HE-DKIM-Result: none/none X-HE-Tag: 1617046128-191432 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Mar 29, 2021 at 11:29:22AM -0700, Roman Gushchin wrote: > On Mon, Mar 29, 2021 at 05:20:55PM +0000, Dennis Zhou wrote: > > On Wed, Mar 24, 2021 at 12:06:23PM -0700, Roman Gushchin wrote: > > > This patch implements partial depopulation of percpu chunks. > > > > > > As now, a chunk can be depopulated only as a part of the final > > > destruction, when there are no more outstanding allocations. However > > > to minimize a memory waste, it might be useful to depopulate a > > > partially filed chunk, if a small number of outstanding allocations > > > prevents the chunk from being reclaimed. > > > > > > This patch implements the following depopulation process: it scans > > > over the chunk pages, looks for a range of empty and populated pages > > > and performs the depopulation. To avoid races with new allocations, > > > the chunk is previously isolated. After the depopulation the chunk is > > > returned to the original slot (but is appended to the tail of the list > > > to minimize the chances of population). > > > > > > Because the pcpu_lock is dropped while calling pcpu_depopulate_chunk(), > > > the chunk can be concurrently moved to a different slot. So we need > > > to isolate it again on each step. pcpu_alloc_mutex is held, so the > > > chunk can't be populated/depopulated asynchronously. > > > > > > Signed-off-by: Roman Gushchin > > > --- > > > mm/percpu.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > 1 file changed, 90 insertions(+) > > > > > > diff --git a/mm/percpu.c b/mm/percpu.c > > > index 6596a0a4286e..78c55c73fa28 100644 > > > --- a/mm/percpu.c > > > +++ b/mm/percpu.c > > > @@ -2055,6 +2055,96 @@ static void __pcpu_balance_workfn(enum pcpu_chunk_type type) > > > mutex_unlock(&pcpu_alloc_mutex); > > > } > > > > > > +/** > > > + * pcpu_shrink_populated - scan chunks and release unused pages to the system > > > + * @type: chunk type > > > + * > > > + * Scan over all chunks, find those marked with the depopulate flag and > > > + * try to release unused pages to the system. On every attempt clear the > > > + * chunk's depopulate flag to avoid wasting CPU by scanning the same > > > + * chunk again and again. > > > + */ > > > +static void pcpu_shrink_populated(enum pcpu_chunk_type type) > > > +{ > > > + struct list_head *pcpu_slot = pcpu_chunk_list(type); > > > + struct pcpu_chunk *chunk; > > > + int slot, i, off, start; > > > + > > > + spin_lock_irq(&pcpu_lock); > > > + for (slot = pcpu_nr_slots - 1; slot >= 0; slot--) { > > > +restart: > > > + list_for_each_entry(chunk, &pcpu_slot[slot], list) { > > > + bool isolated = false; > > > + > > > + if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH) > > > + break; > > > + > > > > Deallocation makes me a little worried for the atomic case as now we > > could in theory pathologically scan deallocated chunks before finding a > > populated one. > > > > I wonder if we should do something like once a chunk gets depopulated, > > it gets deprioritized and then only once we exhaust looking through > > allocated chunks we then find a depopulated chunk and add it back into > > the rotation. Possibly just add another set of slots? I guess it adds a > > few dimensions to pcpu_slots after the memcg change. > > Please, take a look at patch 3 in the series ("percpu: on demand chunk depopulation"). > Chunks considered to be a good target for the depopulation are in advance > marked with a special flag, so we'll actually try to depopulate only > few chunks at once. While the total number of chunks is fairly low, > I think it should work. > > Another option is to link all such chunks into a list and scan over it, > instead of iterating over all slots. > > Adding new dimensions to pcpu_slots is an option too, but I hope we can avoid > this, as it would complicate the code. > Yeah, depopulation has been on the todo list for a while. It adds the dimension/opportunity of bin packing by sidelining chunks and I'm wondering if that is the right thing to do. Do you have a rough idea of the distribution of # of chunks you're seeing? > > > > > + for (i = 0, start = -1; i < chunk->nr_pages; i++) { > > > + if (!chunk->nr_empty_pop_pages) > > > + break; > > > + > > > + /* > > > + * If the page is empty and populated, start or > > > + * extend the [start, i) range. > > > + */ > > > + if (test_bit(i, chunk->populated)) { > > > + off = find_first_bit( > > > + pcpu_index_alloc_map(chunk, i), > > > + PCPU_BITMAP_BLOCK_BITS); > > > + if (off >= PCPU_BITMAP_BLOCK_BITS) { > > > + if (start == -1) > > > + start = i; > > > + continue; > > > + } > > > > Here instead of looking at the alloc_map, you can look at the > > pcpu_block_md and look for a fully free contig_hint. > > Good idea, will try in v2. > > > > > > + } > > > + > > > + /* > > > + * Otherwise check if there is an active range, > > > + * and if yes, depopulate it. > > > + */ > > > + if (start == -1) > > > + continue; > > > + > > > + /* > > > + * Isolate the chunk, so new allocations > > > + * wouldn't be served using this chunk. > > > + * Async releases can still happen. > > > + */ > > > + if (!list_empty(&chunk->list)) { > > > + list_del_init(&chunk->list); > > > + isolated = true; > > > > Maybe when freeing a chunk, we should consider just isolating it period > > and preventing pcpu_free_area() from being able to add the chunk back > > to a pcpu_slot. > > You mean to add a check in pcpu_free_area() if the chunks is isolated? > Yeah, sounds good to me, will do in v2. > Could also be done in pcpu_chunk_relocate() so it's clear an isolated chunk shouldn't be moved. But I think pcpu_free_area() should be the only way the chunk can be moved on the list. > Thank you! > > > > > > + } > > > + > > > + spin_unlock_irq(&pcpu_lock); > > > + pcpu_depopulate_chunk(chunk, start, i); > > > + cond_resched(); > > > + spin_lock_irq(&pcpu_lock); > > > + > > > + pcpu_chunk_depopulated(chunk, start, i); > > > + > > > + /* > > > + * Reset the range and continue. > > > + */ > > > + start = -1; > > > + } > > > + > > > + if (isolated) { > > > + /* > > > + * The chunk could have been moved while > > > + * pcpu_lock wasn't held. Make sure we put > > > + * the chunk back into the slot and restart > > > + * the scanning. > > > + */ > > > + if (list_empty(&chunk->list)) > > > + list_add_tail(&chunk->list, > > > + &pcpu_slot[slot]); > > > + goto restart; > > > + } > > > + } > > > + } > > > + spin_unlock_irq(&pcpu_lock); > > > +} > > > + > > > /** > > > * pcpu_balance_workfn - manage the amount of free chunks and populated pages > > > * @work: unused > > > -- > > > 2.30.2 > > >