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([2a02:810d:4b3f:de9c:642:1aff:fe31:a15c]) by smtp.gmail.com with ESMTPSA id jk14-20020a170903330e00b001b536aaf7c2sm5639127plb.189.2023.06.22.09.41.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 Jun 2023 09:41:25 -0700 (PDT) Message-ID: <26d8fbcf-d34f-0a79-9d91-8c60e66f7341@redhat.com> Date: Thu, 22 Jun 2023 18:41:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v2 14/16] maple_tree: Refine mas_preallocate() node calculations Content-Language: en-US To: "Liam R. Howlett" , Andrew Morton Cc: maple-tree@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Peng Zhang , David Airlie , Boris Brezillon References: <20230612203953.2093911-1-Liam.Howlett@oracle.com> <20230612203953.2093911-15-Liam.Howlett@oracle.com> From: Danilo Krummrich Organization: RedHat In-Reply-To: <20230612203953.2093911-15-Liam.Howlett@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/12/23 22:39, Liam R. Howlett wrote: > Calculate the number of nodes based on the pending write action instead > of assuming the worst case. Liam already gave me a heads-up on this patch, which I already replied to [1]. However, I think it might make sense to also reply to this patch directly. For a mas_preallocate() calculating the actual required nodes to be allocated instead of assuming the worst to work, it is required to ensure that the tree does not change between calling mas_preallocate() and mas_store_prealloc() if my understanding is correct. In DRM however, more specifically the DRM GPUVA Manager [2], we do have the case that we are not able to ensure this: Jobs to create GPU mappings can be submitted by userspace, are queued up by the kernel and are processed asynchronously in dma-fence signalling critical paths, e.g. by using the drm_gpu_scheduler. Hence, we must be able to allocate the worst case amount of node, since at the time a job is submitted we can't predict the state the maple tree keeping track of mappings has once a mapping is inserted in the (asynchronous) dma-fence signalling critical path. A more detailed explanation can be found in [1]. Could we keep a separate function for allocating the worst case amount of nodes additionally to this optimization? E.g. something like mas_preallocate_worst_case() or mas_preallocate_unlocked() (since I guess the new one requires the maple tree to be kept locked in order not to change)? [1] https://lore.kernel.org/nouveau/68cd25de-e767-725e-2e7b-703217230bb0@redhat.com/T/#ma326e200b1de1e3c9df4e9fcb3bf243061fee8b5 [2] https://lore.kernel.org/linux-mm/20230620004217.4700-8-dakr@redhat.com/T/#m47ab82310f87793d0f0cc1825a316eb30ad5b653 - Danilo > > This addresses a performance regression introduced in platforms that > have longer allocation timing. > > Signed-off-by: Liam R. Howlett > --- > lib/maple_tree.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 47 insertions(+), 1 deletion(-) > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c > index 048d6413a114..7ac5b5457603 100644 > --- a/lib/maple_tree.c > +++ b/lib/maple_tree.c > @@ -5541,9 +5541,55 @@ EXPORT_SYMBOL_GPL(mas_store_prealloc); > */ > int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp) > { > + MA_WR_STATE(wr_mas, mas, entry); > + unsigned char node_size; > + int request = 1; > int ret; > > - mas_node_count_gfp(mas, 1 + mas_mt_height(mas) * 3, gfp); > + > + if (unlikely(!mas->index && mas->last == ULONG_MAX)) > + goto ask_now; > + > + mas_wr_store_setup(&wr_mas); > + wr_mas.content = mas_start(mas); > + /* Root expand */ > + if (unlikely(mas_is_none(mas) || mas_is_ptr(mas))) > + goto ask_now; > + > + if (unlikely(!mas_wr_walk(&wr_mas))) { > + /* Spanning store, use worst case for now */ > + request = 1 + mas_mt_height(mas) * 3; > + goto ask_now; > + } > + > + /* At this point, we are at the leaf node that needs to be altered. */ > + /* Exact fit, no nodes needed. */ > + if (wr_mas.r_min == mas->index && wr_mas.r_max == mas->last) > + return 0; > + > + mas_wr_end_piv(&wr_mas); > + node_size = mas_wr_new_end(&wr_mas); > + /* Slot store can avoid using any nodes */ > + if (node_size == wr_mas.node_end && wr_mas.offset_end - mas->offset == 1) > + return 0; > + > + if (node_size >= mt_slots[wr_mas.type]) { > + /* Split, worst case for now. */ > + request = 1 + mas_mt_height(mas) * 2; > + goto ask_now; > + } > + > + /* Appending does not need any nodes */ > + if (node_size == wr_mas.node_end + 1 && mas->offset == wr_mas.node_end) > + return 0; > + > + /* Potential spanning rebalance collapsing a node, use worst-case */ > + if (node_size - 1 <= mt_min_slots[wr_mas.type]) > + request = mas_mt_height(mas) * 2 - 1; > + > + /* node store needs one node */ > +ask_now: > + mas_node_count_gfp(mas, request, gfp); > mas->mas_flags |= MA_STATE_PREALLOC; > if (likely(!mas_is_err(mas))) > return 0;