From patchwork Fri Dec 13 13:02:10 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: SeongJae Park X-Patchwork-Id: 11290577 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id CDCD2175D for ; Fri, 13 Dec 2019 20:36:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 81FED246F1 for ; Fri, 13 Dec 2019 20:36:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="na95Bme5" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727324AbfLMNCy (ORCPT ); Fri, 13 Dec 2019 08:02:54 -0500 Received: from mail-pl1-f193.google.com ([209.85.214.193]:35365 "EHLO mail-pl1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726345AbfLMNCy (ORCPT ); Fri, 13 Dec 2019 08:02:54 -0500 Received: by mail-pl1-f193.google.com with SMTP id s10so1232035plp.2; Fri, 13 Dec 2019 05:02:53 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=nLDfxoPJ5VtxV7g2ayKIW9c6W7jyRpbX81WHy5tkNdc=; b=na95Bme5BuXyVngE8myQYMjzMOhLSpJlxbPfCES+KyNqJhN9pBMo7Sek/1TUdEUl6s feKvl8vXkbftx6BWBVumtm7Bxs+WbGFrPCd/n9Bsa5epPzG0T0Xt3yQLwmIqX3EKGi3D L6q1pVNtT3HY2/Fpax5NGTaE1iN8uYtQSCdp0oVPzsvWdPDNhN162tZd95f16Xw8Va0F KUdEehtK0YygYRFhipSdVJ0WmJw5vxeJekMpzsMoXNYkufIYozKkDuIUeIcTVXUfPtcW dgUeC+UFWFTnJ9MfuPhANerWvCEV5s/Nk0IllLkd6lZJqbncfoi0BaJwM651/UIhPnIi OHvw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=nLDfxoPJ5VtxV7g2ayKIW9c6W7jyRpbX81WHy5tkNdc=; b=CBRY1ELhKtNqLtmLb7F+EzHu9kdYXYXfDlmEH+/mzX4r1oYliPeSyCjmvGJx2mw135 NWNszRofker9438NMwd5R1ZgotIHl4OV/sWuDczWEd7iWLEjoVQidjPG1kKX5BvEb6pi ucUqCnTbT/HGfSEiJQAMZ5kQTBQxQaiqUgtmtYKQ95nLVTl1/1Q9sXq23AoObh5nnW6t nNmilDjnES4huZo1ADp4quHDG2+FKgsU+EU+rYaK3yd2qPjIWVQQWLk06rKA7F6CLH8i RO41ZfRIJU1QWJg1pGoqYb4qVHm0fBqlTA33Fz/i3of38YUdVpKdnTOUf+b5rqBOYOTb QTHw== X-Gm-Message-State: APjAAAXjR6j5ilgRJzbIwm+nAqKLbaZbWDqxG7MVBns1z1DDOCuXuaOy cwDvNGVZOPpyuRiCUvskyVA= X-Google-Smtp-Source: APXvYqwfNs3rYNCe85uoLP4qaAjVzfZujTE+Gw7cyfzke+ypRyumEQSr/Pa7zGuzaOUuJPWIC0vuSw== X-Received: by 2002:a17:90a:200d:: with SMTP id n13mr16663435pjc.16.1576242172888; Fri, 13 Dec 2019 05:02:52 -0800 (PST) Received: from localhost.localdomain ([12.176.148.120]) by smtp.gmail.com with ESMTPSA id k3sm10872278pgc.3.2019.12.13.05.02.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Dec 2019 05:02:52 -0800 (PST) From: SeongJae Park X-Google-Original-From: SeongJae Park To: jgross@suse.com, axboe@kernel.dk, konrad.wilk@oracle.com, roger.pau@citrix.com Cc: SeongJae Park , pdurrant@amazon.com, sjpark@amazon.com, sj38.park@gmail.com, xen-devel@lists.xenproject.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v8 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected Date: Fri, 13 Dec 2019 13:02:10 +0000 Message-Id: <20191213130211.24011-3-sjpark@amazon.de> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20191213130211.24011-1-sjpark@amazon.de> References: <20191213130211.24011-1-sjpark@amazon.de> MIME-Version: 1.0 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Each `blkif` has a free pages pool for the grant mapping. The size of the pool starts from zero and is increased on demand while processing the I/O requests. If current I/O requests handling is finished or 100 milliseconds has passed since last I/O requests handling, it checks and shrinks the pool to not exceed the size limit, `max_buffer_pages`. Therefore, host administrators can cause memory pressure in blkback by attaching a large number of block devices and inducing I/O. Such problematic situations can be avoided by limiting the maximum number of devices that can be attached, but finding the optimal limit is not so easy. Improper set of the limit can results in memory pressure or a resource underutilization. This commit avoids such problematic situations by squeezing the pools (returns every free page in the pool to the system) for a while (users can set this duration via a module parameter) if memory pressure is detected. Discussions =========== The `blkback`'s original shrinking mechanism returns only pages in the pool which are not currently be used by `blkback` to the system. In other words, the pages that are not mapped with granted pages. Because this commit is changing only the shrink limit but still uses the same freeing mechanism it does not touch pages which are currently mapping grants. Once memory pressure is detected, this commit keeps the squeezing limit for a user-specified time duration. The duration should be neither too long nor too short. If it is too long, the squeezing incurring overhead can reduce the I/O performance. If it is too short, `blkback` will not free enough pages to reduce the memory pressure. This commit sets the value as `10 milliseconds` by default because it is a short time in terms of I/O while it is a long time in terms of memory operations. Also, as the original shrinking mechanism works for at least every 100 milliseconds, this could be a somewhat reasonable choice. I also tested other durations (refer to the below section for more details) and confirmed that 10 milliseconds is the one that works best with the test. That said, the proper duration depends on actual configurations and workloads. That's why this commit allows users to set the duration as a module parameter. Memory Pressure Test ==================== To show how this commit fixes the memory pressure situation well, I configured a test environment on a xen-running virtualization system. On the `blkfront` running guest instances, I attach a large number of network-backed volume devices and induce I/O to those. Meanwhile, I measure the number of pages that swapped in (pswpin) and out (pswpout) on the `blkback` running guest. The test ran twice, once for the `blkback` before this commit and once for that after this commit. As shown below, this commit has dramatically reduced the memory pressure: pswpin pswpout before 76,672 185,799 after 212 3,325 Optimal Aggressive Shrinking Duration ------------------------------------- To find a best squeezing duration, I repeated the test with three different durations (1ms, 10ms, and 100ms). The results are as below: duration pswpin pswpout 1 852 6,424 10 212 3,325 100 203 3,340 As expected, the memory pressure has decreased as the duration is increased, but the reduction stopped from the `10ms`. Based on this results, I chose the default duration as 10ms. Performance Overhead Test ========================= This commit could incur I/O performance degradation under severe memory pressure because the squeezing will require more page allocations per I/O. To show the overhead, I artificially made a worst-case squeezing situation and measured the I/O performance of a `blkfront` running guest. For the artificial squeezing, I set the `blkback.max_buffer_pages` using the `/sys/module/xen_blkback/parameters/max_buffer_pages` file. In this test, I set the value to `1024` and `0`. The `1024` is the default value. Setting the value as `0` is same to a situation doing the squeezing always (worst-case). For the I/O performance measurement, I run a simple `dd` command 5 times as below and collect the 'MB/s' results. $ for i in {1..5}; do dd if=/dev/zero of=file \ bs=4k count=$((256*512)); sync; done If the underlying block device is slow enough, the squeezing overhead could be hidden. For the reason, I do this test for both a slow block device and a fast block device. I use a popular cloud block storage service, ebs[1] as a slow device and the ramdisk block device[2] for the fast device. The results are as below. 'max_pgs' represents the value of the `blkback.max_buffer_pages` parameter. On the slow block device ------------------------ max_pgs Min Max Median Avg Stddev 0 38.7 45.8 38.7 40.12 3.1752165 1024 38.7 45.8 38.7 40.12 3.1752165 No difference proven at 95.0% confidence On the fast block device ------------------------ max_pgs Min Max Median Avg Stddev 0 417 423 420 419.4 2.5099801 1024 414 425 416 417.8 4.4384682 No difference proven at 95.0% confidence In short, even worst case squeezing on ramdisk based fast block device makes no visible performance degradation. Please note that this is just a very simple and minimal test. On systems using super-fast block devices and a special I/O workload, the results might be different. If you have any doubt, test on your machine with your workload to find the optimal squeezing duration for you. [1] https://aws.amazon.com/ebs/ [2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html Reviewed-by: Juergen Gross Signed-off-by: SeongJae Park --- .../ABI/testing/sysfs-driver-xen-blkback | 9 ++++++++ drivers/block/xen-blkback/blkback.c | 22 +++++++++++++++++-- drivers/block/xen-blkback/common.h | 2 ++ drivers/block/xen-blkback/xenbus.c | 11 +++++++++- 4 files changed, 41 insertions(+), 3 deletions(-) diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback index 4e7babb3ba1f..a74a6d513c9f 100644 --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback @@ -25,3 +25,12 @@ Description: allocated without being in use. The time is in seconds, 0 means indefinitely long. The default is 60 seconds. + +What: /sys/module/xen_blkback/parameters/buffer_squeeze_duration_ms +Date: December 2019 +KernelVersion: 5.5 +Contact: Roger Pau Monné +Description: + How long the block backend buffers release every free pages in + those under memory pressure. The time is in milliseconds. + The default is 10 milliseconds. diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index fd1e19f1a49f..26606c4896fd 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) HZ * xen_blkif_pgrant_timeout); } +/* Once a memory pressure is detected, squeeze free page pools for a while. */ +static unsigned int buffer_squeeze_duration_ms = 10; +module_param_named(buffer_squeeze_duration_ms, + buffer_squeeze_duration_ms, int, 0644); +MODULE_PARM_DESC(buffer_squeeze_duration_ms, +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); + +static unsigned long buffer_squeeze_end; + +void xen_blkbk_update_buffer_squeeze_end(struct xen_blkif *blkif) +{ + blkif->buffer_squeeze_end = jiffies + + msecs_to_jiffies(buffer_squeeze_duration_ms); +} + static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page) { unsigned long flags; @@ -656,8 +671,11 @@ int xen_blkif_schedule(void *arg) ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL); } - /* Shrink if we have more than xen_blkif_max_buffer_pages */ - shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); + /* Shrink the free pages pool if it is too large. */ + if (time_before(jiffies, buffer_squeeze_end)) + shrink_free_pagepool(ring, 0); + else + shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); if (log_stats && time_after(jiffies, ring->st_print)) print_stats(ring); diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h index 1d3002d773f7..ba653126177d 100644 --- a/drivers/block/xen-blkback/common.h +++ b/drivers/block/xen-blkback/common.h @@ -319,6 +319,7 @@ struct xen_blkif { /* All rings for this device. */ struct xen_blkif_ring *rings; unsigned int nr_rings; + unsigned long buffer_squeeze_end; }; struct seg_buf { @@ -383,6 +384,7 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id); int xen_blkif_schedule(void *arg); int xen_blkif_purge_persistent(void *arg); void xen_blkbk_free_caches(struct xen_blkif_ring *ring); +void xen_blkbk_update_buffer_squeeze_end(struct xen_blkif *blkif); int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt, struct backend_info *be, int state); diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c index b90dbcd99c03..09fe6cb5c4ea 100644 --- a/drivers/block/xen-blkback/xenbus.c +++ b/drivers/block/xen-blkback/xenbus.c @@ -824,6 +824,14 @@ static void frontend_changed(struct xenbus_device *dev, } +void xen_blkbk_reclaim_memory(struct xenbus_device *dev) +{ + struct backend_info *be = dev_get_drvdata(&dev->dev); + + xen_blkbk_update_buffer_squeeze_end(be->blkif); +} + + /* ** Connection ** */ @@ -1115,7 +1123,8 @@ static struct xenbus_driver xen_blkbk_driver = { .ids = xen_blkbk_ids, .probe = xen_blkbk_probe, .remove = xen_blkbk_remove, - .otherend_changed = frontend_changed + .otherend_changed = frontend_changed, + .reclaim_memory = xen_blkbk_reclaim_memory, }; int xen_blkif_xenbus_init(void)