Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754800AbdC2IZh (ORCPT ); Wed, 29 Mar 2017 04:25:37 -0400 Received: from mail.kernel.org ([198.145.29.136]:36428 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754666AbdC2IZW (ORCPT ); Wed, 29 Mar 2017 04:25:22 -0400 Date: Wed, 29 Mar 2017 17:25:10 +0900 From: Masami Hiramatsu To: Ingo Molnar Cc: Steven Rostedt , Ingo Molnar , Alban Crequy , Alban Crequy , Alexei Starovoitov , Jonathan Corbet , Arnaldo Carvalho de Melo , Omar Sandoval , linux-doc@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, iago@kinvolk.io, michael@kinvolk.io, Dorau Lukasz , systemtap@sourceware.org Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty Message-Id: <20170329172510.e012406497fd38a54d5069b3@kernel.org> In-Reply-To: <20170329063005.GA12220@gmail.com> References: <149076484118.24574.7083269903420611708.stgit@devbox> <149076498222.24574.679546540523044200.stgit@devbox> <20170329063005.GA12220@gmail.com> X-Mailer: Sylpheed 3.5.0 (GTK+ 2.24.30; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4113 Lines: 108 On Wed, 29 Mar 2017 08:30:05 +0200 Ingo Molnar wrote: > > * Masami Hiramatsu wrote: > > > @@ -1824,6 +1823,30 @@ void unregister_jprobes(struct jprobe **jps, int num) > > EXPORT_SYMBOL_GPL(unregister_jprobes); > > > > #ifdef CONFIG_KRETPROBES > > + > > +/* Try to use free instance first, if failed, try to allocate new instance */ > > +struct kretprobe_instance *kretprobe_alloc_instance(struct kretprobe *rp) > > +{ > > + struct kretprobe_instance *ri = NULL; > > + unsigned long flags = 0; > > + > > + raw_spin_lock_irqsave(&rp->lock, flags); > > + if (!hlist_empty(&rp->free_instances)) { > > + ri = hlist_entry(rp->free_instances.first, > > + struct kretprobe_instance, hlist); > > + hlist_del(&ri->hlist); > > + } > > + raw_spin_unlock_irqrestore(&rp->lock, flags); > > + > > + /* Populate max active instance if possible */ > > + if (!ri && rp->maxactive < KRETPROBE_MAXACTIVE_ALLOC) { > > + ri = kmalloc(sizeof(*ri) + rp->data_size, GFP_ATOMIC); > > + if (ri) > > + rp->maxactive++; > > + } > > + > > + return ri; > > +} > > /* > > * This kprobe pre_handler is registered with every kretprobe. When probe > > * hits it will set up the return probe. > > @@ -1846,14 +1869,8 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs) > > } > > > > /* TODO: consider to only swap the RA after the last pre_handler fired */ > > - hash = hash_ptr(current, KPROBE_HASH_BITS); > > - raw_spin_lock_irqsave(&rp->lock, flags); > > - if (!hlist_empty(&rp->free_instances)) { > > - ri = hlist_entry(rp->free_instances.first, > > - struct kretprobe_instance, hlist); > > - hlist_del(&ri->hlist); > > - raw_spin_unlock_irqrestore(&rp->lock, flags); > > - > > + ri = kretprobe_alloc_instance(rp); > > + if (ri) { > > ri->rp = rp; > > ri->task = current; > > > > @@ -1868,13 +1885,13 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs) > > > > /* XXX(hch): why is there no hlist_move_head? */ > > INIT_HLIST_NODE(&ri->hlist); > > + hash = hash_ptr(current, KPROBE_HASH_BITS); > > kretprobe_table_lock(hash, &flags); > > hlist_add_head(&ri->hlist, &kretprobe_inst_table[hash]); > > kretprobe_table_unlock(hash, &flags); > > - } else { > > + } else > > rp->nmissed++; > > - raw_spin_unlock_irqrestore(&rp->lock, flags); > > - } > > + > > return 0; > > } > > NOKPROBE_SYMBOL(pre_handler_kretprobe); > > So this is something I missed while the original code was merged, but the concept > looks a bit weird: why do we do any "allocation" while a handler is executing? > > That's fundamentally fragile. What's the maximum number of parallel > 'kretprobe_instance' required per kretprobe - one per CPU? It depends on the place where we put the probe. If the probed function will be blocked (yield to other tasks), then we need a same number of threads on the system which can invoke the function. So, ultimately, it is same as function_graph tracer, we need it for each thread. > > If so then we should preallocate all of them when they are installed and not do > any alloc/free dance when executing them. > > This will also speed them up, and increase robustness all around. I see, kretprobe already do that, and I keep it on the code. By default, kretprobe will allocate NR_CPU of kretprobe_instance for each kretprobe. For usual usecase (deeper inside functions in kernel) that is OK. However, as Lukasz reported, for the function near the syscall entry, it may require more instances. In that case, kretprobe user needs to increase maxactive before registering kretprobes, which will be done by Alban's patch. However, the next question is, how many instances are actually needed. User may have to do trial & error loop to find that. For professional users, they will do that, but for the light users, they may not want to do that. I'm also considering to provide a "knob" of disabing this dynamic allocation feature on debugfs, which will help users who would like to avoid memory allocation on kretprobe. Thank you, -- Masami Hiramatsu