From mboxrd@z Thu Jan 1 00:00:00 1970 From: catalin.marinas@arm.com (Catalin Marinas) Date: Wed, 4 Nov 2015 14:54:46 +0000 Subject: [PATCH] arm64: Increase the max granular size In-Reply-To: References: <1442944788-17254-1-git-send-email-rric@kernel.org> <20151028190948.GJ8899@e104818-lin.cambridge.arm.com> <20151103120504.GF7637@e104818-lin.cambridge.arm.com> <20151103143858.GI7637@e104818-lin.cambridge.arm.com> <20151103185050.GJ7637@e104818-lin.cambridge.arm.com> <20151104123640.GK7637@e104818-lin.cambridge.arm.com> Message-ID: <20151104145445.GL7637@e104818-lin.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Nov 04, 2015 at 07:53:50AM -0600, Christoph Lameter wrote: > On Wed, 4 Nov 2015, Catalin Marinas wrote: > > > The simplest option would be to make sure that off slab isn't allowed > > for caches of KMALLOC_MIN_SIZE or smaller, with the drawback that not > > only "kmalloc-128" but any other such caches will be on slab. > > The reason for an off slab configuration is denser object packing. > > > I think a better option would be to first check that there is a > > kmalloc_caches[] entry for freelist_size before deciding to go off-slab. > > Hmmm.. Yes seems to be an option. > > Maybe we simply revert commit 8fc9cf420b36 instead? I'm fine with this. Also note that the arm64 commit changing L1_CACHE_BYTES to 128 hasn't been pushed yet (it's queued for 4.4). > That does not seem to make too much sense to me and the goal of the > commit cannot be accomplished on ARM. Your patch essentially reverts > the effect anyways. In theory it only reverts the effect for the first kmalloc_cache ("kmalloc-128" in the arm64 case). Any other bigger cache which would not be mergeable with an existing one still has the potential of off-slab management. > Smaller slabs really do not need off slab management anyways since they > will only loose a few objects per slab page. IIUC, starting with 128 slab size for a 4KB page, you have 32 objects per page. The freelist takes 32 bytes (or 31), therefore you waste a single slab object. However, only 1/4 of it is used for freelist and the waste gets bigger with 256 slab size, hence the original commit. BTW, assuming L1_CACHE_BYTES is 512 (I don't ever see this happening but just in theory), we potentially have the same issue. What would save us is that INDEX_NODE would match the first "kmalloc-512" cache, so we have it pre-populated. -- Catalin