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[82.30.61.225]) by smtp.gmail.com with ESMTPSA id z6-20020adfec86000000b0022b1d74dc56sm12539927wrn.79.2022.10.04.06.55.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Oct 2022 06:55:12 -0700 (PDT) Date: Tue, 4 Oct 2022 14:55:10 +0100 From: "Dr. David Alan Gilbert" To: Peter Xu Cc: qemu-devel@nongnu.org, Manish Mishra , Juan Quintela , ani@anisinha.ca, Leonardo Bras Soares Passos , "Daniel P . Berrange" Subject: Re: [PATCH 05/14] migration: Yield bitmap_mutex properly when sending/sleeping Message-ID: References: <20220920225106.48451-1-peterx@redhat.com> <20220920225210.48732-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220920225210.48732-1-peterx@redhat.com> User-Agent: Mutt/2.2.7 (2022-08-07) Received-SPF: pass client-ip=170.10.129.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Peter Xu (peterx@redhat.com) wrote: > Don't take the bitmap mutex when sending pages, or when being throttled by > migration_rate_limit() (which is a bit tricky to call it here in ram code, > but seems still helpful). > > It prepares for the possibility of concurrently sending pages in >1 threads > using the function ram_save_host_page() because all threads may need the > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of > qemu_sem_wait() blocking for one thread will not block the other from > progressing. > > Signed-off-by: Peter Xu I generally dont like taking locks conditionally; but this kind of looks OK; I think it needs a big comment on the start of the function saying that it's called and left with the lock held but that it might drop it temporarily. > --- > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > 1 file changed, 31 insertions(+), 11 deletions(-) > > diff --git a/migration/ram.c b/migration/ram.c > index 8303252b6d..6e7de6087a 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > */ > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > { > + bool page_dirty, release_lock = postcopy_preempt_active(); Could you rename that to something like 'drop_lock' - you are taking the lock at the end even when you have 'release_lock' set - which is a bit strange naming. > int tmppages, pages = 0; > size_t pagesize_bits = > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > break; > } > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > + /* > + * Properly yield the lock only in postcopy preempt mode because > + * both migration thread and rp-return thread can operate on the > + * bitmaps. > + */ > + if (release_lock) { > + qemu_mutex_unlock(&rs->bitmap_mutex); > + } Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > /* Check the pages is dirty and if it is send it */ > - if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) { > + if (page_dirty) { > tmppages = ram_save_target_page(rs, pss); > - if (tmppages < 0) { > - return tmppages; > + if (tmppages >= 0) { > + pages += tmppages; > + /* > + * Allow rate limiting to happen in the middle of huge pages if > + * something is sent in the current iteration. > + */ > + if (pagesize_bits > 1 && tmppages > 0) { > + migration_rate_limit(); This feels interesting, I know it's no change from before, and it's difficult to do here, but it seems odd to hold the lock around the sleeping in the rate limit. Dave > + } > } > + } else { > + tmppages = 0; > + } > > - pages += tmppages; > - /* > - * Allow rate limiting to happen in the middle of huge pages if > - * something is sent in the current iteration. > - */ > - if (pagesize_bits > 1 && tmppages > 0) { > - migration_rate_limit(); > - } > + if (release_lock) { > + qemu_mutex_lock(&rs->bitmap_mutex); > } > + > + if (tmppages < 0) { > + return tmppages; > + } > + > pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page); > } while ((pss->page < hostpage_boundary) && > offset_in_ramblock(pss->block, > -- > 2.32.0 > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK