From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E1312C43461 for ; Fri, 11 Sep 2020 15:57:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F02721D7E for ; Fri, 11 Sep 2020 15:57:52 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="vghDKLe7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726400AbgIKP5s (ORCPT ); Fri, 11 Sep 2020 11:57:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38316 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726504AbgIKP4G (ORCPT ); Fri, 11 Sep 2020 11:56:06 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 48041C061756 for ; Fri, 11 Sep 2020 08:56:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=sLXAoluFDtJ8Zc5CiVOegUS5j1nsglapDNRGh1P5SGM=; b=vghDKLe7g+ASu3Mr4jFooJRrWZ cLrBpF37AcDt0qkkZtTe4SYh0SHzOOqdmBED4KfSt+XTTUAJbvw26wX6DZqTHGzntddRq6QkjyZnz 9JZNKqA9SlETk/er3CWk7UAcJLrp6TBvNfWJPkdrneIZl5UhEFuoL4Yf8kKcbvb6ozV5/0tm5EwdV gRZ+PsFsLqsJSUIIyhMrK9TniKdsanM/P56c4WrjxXjh4V/lEIN2qHMNhMY7DJFoUfUQRcJyPF5lP Ujgn8C1sADw338oLMOZN83YEgb/85m642vX6L7twC1XmiC0tFxpv7CjtU/R9NQvzC2qS7q3c5WjET SJa0k1tw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kGlP2-0001WH-Ck; Fri, 11 Sep 2020 15:55:56 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 87F66303DA0; Fri, 11 Sep 2020 17:55:55 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 69FD52B65D593; Fri, 11 Sep 2020 17:55:55 +0200 (CEST) Date: Fri, 11 Sep 2020 17:55:55 +0200 From: peterz@infradead.org To: qianjun.kernel@gmail.com Cc: tglx@linutronix.de, will@kernel.org, luto@kernel.org, linux-kernel@vger.kernel.org, laoar.shao@gmail.com, urezki@gmail.com, frederic@kernel.org Subject: Re: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs Message-ID: <20200911155555.GX2674@hirez.programming.kicks-ass.net> References: <20200909090931.8836-1-qianjun.kernel@gmail.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="ALfTUftag+2gvp1h" Content-Disposition: inline In-Reply-To: <20200909090931.8836-1-qianjun.kernel@gmail.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --ALfTUftag+2gvp1h Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, Sep 09, 2020 at 05:09:31PM +0800, qianjun.kernel@gmail.com wrote: > From: jun qian > > When get the pending softirqs, it need to process all the pending > softirqs in the while loop. If the processing time of each pending > softirq is need more than 2 msec in this loop, or one of the softirq > will running a long time, according to the original code logic, it > will process all the pending softirqs without wakeuping ksoftirqd, > which will cause a relatively large scheduling delay on the > corresponding CPU, which we do not wish to see. The patch will check > the total time to process pending softirq, if the time exceeds 2 ms > we need to wakeup the ksofirqd to aviod large sched delay. But what is all that unreadaable gibberish with pending_new_{flag,bit} ? Random comments below.. > +#define MAX_SOFTIRQ_TIME_NS 2000000 2*NSEC_PER_MSEC > +DEFINE_PER_CPU(__u32, pending_new_flag); > +DEFINE_PER_CPU(__u32, pending_next_bit); __u32 is for userspace ABI, this is not it, use u32 > +#define SOFTIRQ_PENDING_MASK ((1UL << NR_SOFTIRQS) - 1) > + > asmlinkage __visible void __softirq_entry __do_softirq(void) > { > - unsigned long end = jiffies + MAX_SOFTIRQ_TIME; > + u64 end = sched_clock() + MAX_SOFTIRQ_TIME_NS; > unsigned long old_flags = current->flags; > int max_restart = MAX_SOFTIRQ_RESTART; > struct softirq_action *h; > bool in_hardirq; > - __u32 pending; > - int softirq_bit; > + __u32 pending, pending_left, pending_new; > + int softirq_bit, next_bit; > + unsigned long flags; > > /* > * Mask out PF_MEMALLOC as the current task context is borrowed for the > @@ -277,10 +282,33 @@ asmlinkage __visible void __softirq_entry __do_softirq(void) > > h = softirq_vec; > > - while ((softirq_bit = ffs(pending))) { > - unsigned int vec_nr; > + next_bit = per_cpu(pending_next_bit, smp_processor_id()); > + per_cpu(pending_new_flag, smp_processor_id()) = 0; __this_cpu_read() / __this_cpu_write() > + > + pending_left = pending & > + (SOFTIRQ_PENDING_MASK << next_bit); > + pending_new = pending & > + (SOFTIRQ_PENDING_MASK >> (NR_SOFTIRQS - next_bit)); The second mask is the inverse of the first. > + /* > + * In order to be fair, we shold process the pengding bits by the > + * last processing order. > + */ > + while ((softirq_bit = ffs(pending_left)) || > + (softirq_bit = ffs(pending_new))) { > int prev_count; > + unsigned int vec_nr = 0; > > + /* > + * when the left pengding bits have been handled, we should > + * to reset the h to softirq_vec. > + */ > + if (!ffs(pending_left)) { > + if (per_cpu(pending_new_flag, smp_processor_id()) == 0) { > + h = softirq_vec; > + per_cpu(pending_new_flag, smp_processor_id()) = 1; > + } > + } > h += softirq_bit - 1; > > vec_nr = h - softirq_vec; > @@ -298,17 +326,44 @@ asmlinkage __visible void __softirq_entry __do_softirq(void) > preempt_count_set(prev_count); > } > h++; > - pending >>= softirq_bit; > + > + if (ffs(pending_left)) This is the _third_ ffs(pending_left), those things are _expensive_ (on some archs, see include/asm-generic/bitops/__ffs.h). > + pending_left >>= softirq_bit; > + else > + pending_new >>= softirq_bit; > + > + /* > + * the softirq's action has been run too much time, > + * so it may need to wakeup the ksoftirqd > + */ > + if (need_resched() && sched_clock() > end) { > + /* > + * Ensure that the remaining pending bits will be > + * handled. > + */ > + local_irq_save(flags); > + if (ffs(pending_left)) *fourth*... > + or_softirq_pending((pending_left << (vec_nr + 1)) | > + pending_new); > + else > + or_softirq_pending(pending_new << (vec_nr + 1)); > + local_irq_restore(flags); > + per_cpu(pending_next_bit, smp_processor_id()) = vec_nr + 1; > + break; > + } > } > > + /* reset the pending_next_bit */ > + per_cpu(pending_next_bit, smp_processor_id()) = 0; > + > if (__this_cpu_read(ksoftirqd) == current) > rcu_softirq_qs(); > local_irq_disable(); > > pending = local_softirq_pending(); > if (pending) { > - if (time_before(jiffies, end) && !need_resched() && > - --max_restart) > + if (!need_resched() && --max_restart && > + sched_clock() <= end) > goto restart; > > wakeup_softirqd(); This really wants to be a number of separate patches; and I quickly lost the plot in your code. Instead of cleaning things up, you're making an even bigger mess of things. That said, I _think_ I've managed to decode what you want. See the completely untested patches attached. --ALfTUftag+2gvp1h Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="peterz-softirq-fix-loop.patch" Subject: softirq: Rewrite softirq processing loop From: Peter Zijlstra Date: Fri Sep 11 17:00:03 CEST 2020 Simplify the softirq processing loop by using the bitmap APIs Signed-off-by: Peter Zijlstra (Intel) --- kernel/softirq.c | 16 ++++++---------- 1 file changed, 6 insertions(+), 10 deletions(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -258,9 +258,9 @@ asmlinkage __visible void __softirq_entr unsigned long old_flags = current->flags; int max_restart = MAX_SOFTIRQ_RESTART; struct softirq_action *h; + unsigned long pending; + unsigned int vec_nr; bool in_hardirq; - __u32 pending; - int softirq_bit; /* * Mask out PF_MEMALLOC as the current task context is borrowed for the @@ -281,15 +281,13 @@ asmlinkage __visible void __softirq_entr local_irq_enable(); - h = softirq_vec; + for_each_set_bit(vec_nr, &pending, NR_SOFTIRQS) { + unsigned int prev_count; - while ((softirq_bit = ffs(pending))) { - unsigned int vec_nr; - int prev_count; + __clear_bit(vec_nr, &pending); - h += softirq_bit - 1; + h = softirq_vec + vec_nr; - vec_nr = h - softirq_vec; prev_count = preempt_count(); kstat_incr_softirqs_this_cpu(vec_nr); @@ -303,8 +301,6 @@ asmlinkage __visible void __softirq_entr prev_count, preempt_count()); preempt_count_set(prev_count); } - h++; - pending >>= softirq_bit; } if (__this_cpu_read(ksoftirqd) == current) --ALfTUftag+2gvp1h Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="peterz-softirq-timo.patch" Subject: softirq: Use sched_clock() based timeout From: Peter Zijlstra Date: Fri Sep 11 17:30:01 CEST 2020 Replace the jiffies based timeout with a sched_clock() based one. Signed-off-by: Peter Zijlstra (Intel) --- kernel/softirq.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -25,6 +25,7 @@ #include #include #include +#include #define CREATE_TRACE_POINTS #include @@ -216,7 +217,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip); * we want to handle softirqs as soon as possible, but they * should not be able to lock up the box. */ -#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2) +#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC #define MAX_SOFTIRQ_RESTART 10 #ifdef CONFIG_TRACE_IRQFLAGS @@ -254,9 +255,9 @@ static inline void lockdep_softirq_end(b asmlinkage __visible void __softirq_entry __do_softirq(void) { - unsigned long end = jiffies + MAX_SOFTIRQ_TIME; unsigned long old_flags = current->flags; int max_restart = MAX_SOFTIRQ_RESTART; + u64 start = sched_clock(); struct softirq_action *h; unsigned long pending; unsigned int vec_nr; @@ -309,7 +310,7 @@ asmlinkage __visible void __softirq_entr pending = local_softirq_pending(); if (pending) { - if (time_before(jiffies, end) && !need_resched() && + if (sched_clock() - start < MAX_SOFTIRQ_TIME && !need_resched() && --max_restart) goto restart; --ALfTUftag+2gvp1h Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="peterz-softirq-needs-break.patch" Subject: softirq: Factor loop termination condition From: Peter Zijlstra Date: Fri Sep 11 17:17:20 CEST 2020 Signed-off-by: Peter Zijlstra (Intel) --- kernel/softirq.c | 44 +++++++++++++++++++++++++------------------- 1 file changed, 25 insertions(+), 19 deletions(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -204,22 +204,6 @@ void __local_bh_enable_ip(unsigned long } EXPORT_SYMBOL(__local_bh_enable_ip); -/* - * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times, - * but break the loop if need_resched() is set or after 2 ms. - * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in - * certain cases, such as stop_machine(), jiffies may cease to - * increment and so we need the MAX_SOFTIRQ_RESTART limit as - * well to make sure we eventually return from this method. - * - * These limits have been established via experimentation. - * The two things to balance is latency against fairness - - * we want to handle softirqs as soon as possible, but they - * should not be able to lock up the box. - */ -#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC -#define MAX_SOFTIRQ_RESTART 10 - #ifdef CONFIG_TRACE_IRQFLAGS /* * When we run softirqs from irq_exit() and thus on the hardirq stack we need @@ -253,10 +237,33 @@ static inline bool lockdep_softirq_start static inline void lockdep_softirq_end(bool in_hardirq) { } #endif +/* + * We restart softirq processing but break the loop if need_resched() is set or + * after 2 ms. The MAX_SOFTIRQ_RESTART guarantees a loop termination if + * sched_clock() were ever to stall. + * + * These limits have been established via experimentation. The two things to + * balance is latency against fairness - we want to handle softirqs as soon as + * possible, but they should not be able to lock up the box. + */ +#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC +#define MAX_SOFTIRQ_RESTART 10 + +static inline bool __softirq_needs_break(u64 start) +{ + if (need_resched()) + return true; + + if (sched_clock() - start >= MAX_SOFTIRQ_TIME) + return true; + + return false; +} + asmlinkage __visible void __softirq_entry __do_softirq(void) { + unsigned int max_restart = MAX_SOFTIRQ_RESTART; unsigned long old_flags = current->flags; - int max_restart = MAX_SOFTIRQ_RESTART; u64 start = sched_clock(); struct softirq_action *h; unsigned long pending; @@ -310,8 +317,7 @@ asmlinkage __visible void __softirq_entr pending = local_softirq_pending(); if (pending) { - if (sched_clock() - start < MAX_SOFTIRQ_TIME && !need_resched() && - --max_restart) + if (!__softirq_needs_break(start) && --max_restart) goto restart; wakeup_softirqd(); --ALfTUftag+2gvp1h Content-Type: text/x-diff; charset=us-ascii Content-Disposition: attachment; filename="peterz-softirq-break-more.patch" Subject: softirq: Allow early break From: Peter Zijlstra Date: Fri Sep 11 17:50:17 CEST 2020 Allow terminating the softirq processing loop without finishing the vectors. Signed-off-by: Peter Zijlstra (Intel) --- kernel/softirq.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -309,19 +309,23 @@ asmlinkage __visible void __softirq_entr prev_count, preempt_count()); preempt_count_set(prev_count); } + + if (pending && __softirq_needs_break(start)) + break; } if (__this_cpu_read(ksoftirqd) == current) rcu_softirq_qs(); local_irq_disable(); - pending = local_softirq_pending(); - if (pending) { - if (!__softirq_needs_break(start) && --max_restart) - goto restart; + if (pending) + or_softirq_pending(pending); + else if ((pending = local_softirq_pending()) && + !__softirq_needs_break(start) && + --max_restart) + goto restart; - wakeup_softirqd(); - } + wakeup_softirqd(); lockdep_softirq_end(in_hardirq); account_irq_exit_time(current); --ALfTUftag+2gvp1h--