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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 61631C0015E for ; Mon, 24 Jul 2023 08:43:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231787AbjGXInm (ORCPT ); Mon, 24 Jul 2023 04:43:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41672 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231765AbjGXIn0 (ORCPT ); Mon, 24 Jul 2023 04:43:26 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 90C3F1B4; Mon, 24 Jul 2023 01:43:08 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 2EC4C60FD3; Mon, 24 Jul 2023 08:43:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E4C9AC433C7; Mon, 24 Jul 2023 08:43:06 +0000 (UTC) Date: Mon, 24 Jul 2023 04:43:04 -0400 From: Steven Rostedt To: Zheng Yejian Cc: , , , Subject: Re: [PATCH] ring-buffer: Fix wrong stat of cpu_buffer->read Message-ID: <20230724044304.5a93a266@rorschach.local.home> In-Reply-To: <20230724054040.3489499-1-zhengyejian1@huawei.com> References: <20230724054040.3489499-1-zhengyejian1@huawei.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Do you folks find it fun to post a fix right after I send Linus a pull request? ;-) Thanks, I'll queue it up (but I will wait a bit before posting in case more fixes trickle in) -- Steve On Mon, 24 Jul 2023 13:40:40 +0800 Zheng Yejian wrote: > When pages are removed in rb_remove_pages(), 'cpu_buffer->read' is set > to 0 in order to make sure any read iterators reset themselves. However, > this will mess 'entries' stating, see following steps: > > # cd /sys/kernel/tracing/ > # 1. Enlarge ring buffer prepare for later reducing: > # echo 20 > per_cpu/cpu0/buffer_size_kb > # 2. Write a log into ring buffer of cpu0: > # taskset -c 0 echo "hello1" > trace_marker > # 3. Read the log: > # cat per_cpu/cpu0/trace_pipe > <...>-332 [000] ..... 62.406844: tracing_mark_write: hello1 > # 4. Stop reading and see the stats, now 0 entries, and 1 event readed: > # cat per_cpu/cpu0/stats > entries: 0 > [...] > read events: 1 > # 5. Reduce the ring buffer > # echo 7 > per_cpu/cpu0/buffer_size_kb > # 6. Now entries became unexpected 1 because actually no entries!!! > # cat per_cpu/cpu0/stats > entries: 1 > [...] > read events: 0 > > To fix it, introduce 'page_removed' field to count total removed pages > since last reset, then use it to let read iterators reset themselves > instead of changing the 'read' pointer. > > Fixes: 83f40318dab0 ("ring-buffer: Make removal of ring buffer pages atomic") > Signed-off-by: Zheng Yejian > --- > kernel/trace/ring_buffer.c | 22 ++++++++++++---------- > 1 file changed, 12 insertions(+), 10 deletions(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index de061dd47313..46b4a3c7c3bf 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -523,6 +523,8 @@ struct ring_buffer_per_cpu { > rb_time_t before_stamp; > u64 event_stamp[MAX_NEST]; > u64 read_stamp; > + /* pages removed since last reset */ > + unsigned long pages_removed; > /* ring buffer pages to update, > 0 to add, < 0 to remove */ > long nr_pages_to_update; > struct list_head new_pages; /* new pages to add */ > @@ -559,6 +561,7 @@ struct ring_buffer_iter { > struct buffer_page *head_page; > struct buffer_page *cache_reader_page; > unsigned long cache_read; > + unsigned long cache_pages_removed; > u64 read_stamp; > u64 page_stamp; > struct ring_buffer_event *event; > @@ -1957,6 +1960,8 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) > to_remove = rb_list_head(to_remove)->next; > head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD; > } > + /* Read iterators need to reset themselves when some pages removed */ > + cpu_buffer->pages_removed += nr_removed; > > next_page = rb_list_head(to_remove)->next; > > @@ -1978,12 +1983,6 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) > cpu_buffer->head_page = list_entry(next_page, > struct buffer_page, list); > > - /* > - * change read pointer to make sure any read iterators reset > - * themselves > - */ > - cpu_buffer->read = 0; > - > /* pages are removed, resume tracing and then free the pages */ > atomic_dec(&cpu_buffer->record_disabled); > raw_spin_unlock_irq(&cpu_buffer->reader_lock); > @@ -4395,6 +4394,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter) > > iter->cache_reader_page = iter->head_page; > iter->cache_read = cpu_buffer->read; > + iter->cache_pages_removed = cpu_buffer->pages_removed; > > if (iter->head) { > iter->read_stamp = cpu_buffer->read_stamp; > @@ -4849,12 +4849,13 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts) > buffer = cpu_buffer->buffer; > > /* > - * Check if someone performed a consuming read to > - * the buffer. A consuming read invalidates the iterator > - * and we need to reset the iterator in this case. > + * Check if someone performed a consuming read to the buffer > + * or removed some pages from the buffer. In these cases, > + * iterator was invalidated and we need to reset it. > */ > if (unlikely(iter->cache_read != cpu_buffer->read || > - iter->cache_reader_page != cpu_buffer->reader_page)) > + iter->cache_reader_page != cpu_buffer->reader_page || > + iter->cache_pages_removed != cpu_buffer->pages_removed)) > rb_iter_reset(iter); > > again: > @@ -5298,6 +5299,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer) > cpu_buffer->last_overrun = 0; > > rb_head_page_activate(cpu_buffer); > + cpu_buffer->pages_removed = 0; > } > > /* Must have disabled the cpu buffer then done a synchronize_rcu */