From: John Johansen <firstname.lastname@example.org> To: Sebastian Andrzej Siewior <email@example.com> Cc: firstname.lastname@example.org, James Morris <email@example.com>, "Serge E. Hallyn" <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches Date: Wed, 1 May 2019 14:29:17 -0700 Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On 4/30/19 7:47 AM, Sebastian Andrzej Siewior wrote: > On 2019-04-28 16:56:59 [-0700], John Johansen wrote: >> So digging into why the history of the per cpu buffers in apparmor. >> We used to do buffer allocations via kmalloc and there were a few reasons >> for the switch >> >> * speed/lockless: speaks for it self, mediation is already slow enough > > it is shared among all CPUs but it is a small/quick operation to > add/return a buffer. > I wouldn't exactly call taking a lock speedy. Getting an available buffer or returning it is indeed quick. The allocation fall back not so much. >> * some buffer allocations had to be done with GFP_ATOMIC, making them >> more likely to fail. Since we fail closed that means failure would >> block access. This actually became a serious problem in a couple >> places. Switching to per cpu buffers and blocking pre-empt was >> the solution. > > GFP_KERNEL is allowed to use IO/SWAP and ATOMIC has emergency pools. The > new approach won't return a NULL pointer, simply spin to either allocate > new memory or get one which was just returned. > yeah, I am not really a fan of a potential infinite loop trying to allocate memory. It may be worth retrying once or twice but potentially infinitely spinning on failed allocation really isn't acceptable. >> * in heavy use cases we would see a lot of buffers being allocated >> and freed. Which resulted in locking slow downs and also buffer >> allocation failures. So having the buffers preallocated allowed us >> to bound this potential problem. >> >> This was all 6 years ago. Going to a mem pool certainly could help, >> reduce the memory foot print, and would definitely help with >> preempt/real time kernels. >> >> A big concern with this patchset is reverting back to GFP_KERNEL >> for everything. We definitely were getting failures due to allocations >> in atomic context. There have been lots of changes in the kernel over >> the last six years so it possible these cases don't exist anymore. I >> went through and built some kernels with this patchset and have run >> through some testing without tripping that problem but I don't think >> it has seen enough testing yet. > > Do you want apply #1 now and #2 later? I audited the ATOMIC->KERNEL > changes manually and I didn't see any atomic context. It looked like the > only reason for ATOMIC was the preempt_disable() due to the memory pool. > Indeed most if not all (I'd have to dig to be sure) the changes made in #2 were original done because of the move to the per cpu buffers and blocking pre-emption. The problem was with the allocation of the buffer needing to be GFP_ATOMIC some times.
next prev parent reply index Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-04-05 13:34 Sebastian Andrzej Siewior 2019-04-05 13:34 ` [PATCH 2/2] apparmor: Switch to GFP_KERNEL where possible Sebastian Andrzej Siewior 2019-05-07 19:57 ` John Johansen 2019-04-15 10:50 ` [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches Sebastian Andrzej Siewior 2019-04-28 23:56 ` John Johansen 2019-04-30 14:47 ` Sebastian Andrzej Siewior 2019-05-01 21:29 ` John Johansen [this message] 2019-05-02 10:51 ` Sebastian Andrzej Siewior 2019-05-02 13:17 ` Tetsuo Handa 2019-05-02 13:47 ` Sebastian Andrzej Siewior 2019-05-02 14:10 ` Tetsuo Handa 2019-05-03 11:48 ` [PATCH v2] " Sebastian Andrzej Siewior 2019-05-03 11:51 ` [PATCH v3] " Sebastian Andrzej Siewior 2019-05-03 12:41 ` Tetsuo Handa 2019-05-03 14:12 ` [PATCH v4] " Sebastian Andrzej Siewior 2019-05-07 19:57 ` John Johansen 2019-10-02 8:59 ` Sebastian Andrzej Siewior 2019-10-02 15:47 ` John Johansen 2019-10-02 15:52 ` Sebastian Andrzej Siewior 2019-05-02 19:33 ` [PATCH 1/2] " John Johansen
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