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Message-ID: <c99a0d82-e76d-4ed8-afc3-e26be5497ba2@suse.de>
Date: Tue, 16 Jul 2024 08:37:47 +0200
From: Hannes Reinecke <hare@...e.de>
To: Stuart Hayes <stuart.w.hayes@...il.com>, linux-kernel@...r.kernel.org,
Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
linux-nvme@...ts.infradead.org
Cc: Martin Wilck <martin.wilck@...e.com>,
Ayush Siddarath <ayush.siddarath@...l.com>
Subject: Re: [PATCH v3] nvme_core: scan namespaces asynchronously
On 7/15/24 22:34, Stuart Hayes wrote:
> Use async function calls to make namespace scanning happen in parallel.
>
> Without the patch, NVME namespaces are scanned serially, so it can take
> a long time for all of a controller's namespaces to become available,
> especially with a slower (TCP) interface with large number of
> namespaces.
>
> It is not uncommon to have large numbers (hundreds or thousands) of
> namespaces on nvme-of with storage servers.
>
> The time it took for all namespaces to show up after connecting (via
> TCP) to a controller with 1002 namespaces was measured on one system:
>
> network latency without patch with patch
> 0 6s 1s
> 50ms 210s 10s
> 100ms 417s 18s
>
> Measurements taken on another system show the effect of the patch on the
> time nvme_scan_work() took to complete, when connecting to a linux
> nvme-of target with varying numbers of namespaces, on a network of
> 400us.
>
> namespaces without patch with patch
> 1 16ms 14ms
> 2 24ms 16ms
> 4 49ms 22ms
> 8 101ms 33ms
> 16 207ms 56ms
> 100 1.4s 0.6s
> 1000 12.9s 2.0s
>
> On the same system, connecting to a local PCIe NVMe drive (a Samsung
> PM1733) instead of a network target:
>
> namespaces without patch with patch
> 1 13ms 12ms
> 2 41ms 13ms
>
> Signed-off-by: Stuart Hayes <stuart.w.hayes@...il.com>
> ---
> changes from V2:
> * make a separate function nvme_scan_ns_async() that calls
> nvme_scan_ns(), instead of modifying nvme_scan_ns()
> * only scan asynchronously from nvme_scan_ns_list(), not from
> nvme_scan_ns_sequential()
> * provide more timing data in the commit message
>
> changes from V1:
> * remove module param to enable/disable async scanning
> * add scan time measurements to commit message
>
>
> drivers/nvme/host/core.c | 48 +++++++++++++++++++++++++++++++++-------
> 1 file changed, 40 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 782090ce0bc1..dbf05cfea063 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -4,6 +4,7 @@
> * Copyright (c) 2011-2014, Intel Corporation.
> */
>
> +#include <linux/async.h>
> #include <linux/blkdev.h>
> #include <linux/blk-mq.h>
> #include <linux/blk-integrity.h>
> @@ -3952,6 +3953,30 @@ static void nvme_scan_ns(struct nvme_ctrl *ctrl, unsigned nsid)
> }
> }
>
> +/*
> + * struct async_scan_info - keeps track of controller & NSIDs to scan
> + * @ctrl: Controller on which namespaces are being scanned
> + * @next_idx: Index of next NSID to scan in ns_list
> + * @ns_list: Pointer to list of NSIDs to scan
> + */
> +struct async_scan_info {
> + struct nvme_ctrl *ctrl;
> + atomic_t next_idx;
> + __le32 *ns_list;
> +};
> +
> +static void nvme_scan_ns_async(void *data, async_cookie_t cookie)
> +{
> + struct async_scan_info *scan_info = data;
> + int idx;
> + u32 nsid;
> +
> + idx = (u32)atomic_fetch_add(1, &scan_info->next_idx);
> + nsid = le32_to_cpu(scan_info->ns_list[idx]);
> +
> + nvme_scan_ns(scan_info->ctrl, nsid);
> +}
> +
> static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
> unsigned nsid)
> {
> @@ -3975,12 +4000,14 @@ static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
> static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
> {
> const int nr_entries = NVME_IDENTIFY_DATA_SIZE / sizeof(__le32);
> - __le32 *ns_list;
> + struct async_scan_info scan_info;
> u32 prev = 0;
> int ret = 0, i;
> + ASYNC_DOMAIN(domain);
>
> - ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
> - if (!ns_list)
> + scan_info.ctrl = ctrl;
> + scan_info.ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
> + if (!scan_info.ns_list)
> return -ENOMEM;
>
> for (;;) {
> @@ -3990,28 +4017,33 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
> .identify.nsid = cpu_to_le32(prev),
> };
>
> - ret = nvme_submit_sync_cmd(ctrl->admin_q, &cmd, ns_list,
> - NVME_IDENTIFY_DATA_SIZE);
> + ret = nvme_submit_sync_cmd(ctrl->admin_q, &cmd,
> + scan_info.ns_list,
> + NVME_IDENTIFY_DATA_SIZE);
> if (ret) {
> dev_warn(ctrl->device,
> "Identify NS List failed (status=0x%x)\n", ret);
> goto free;
> }
>
> + atomic_set(&scan_info.next_idx, 0);
> for (i = 0; i < nr_entries; i++) {
> - u32 nsid = le32_to_cpu(ns_list[i]);
> + u32 nsid = le32_to_cpu(scan_info.ns_list[i]);
>
> if (!nsid) /* end of the list? */
> goto out;
> - nvme_scan_ns(ctrl, nsid);
> + async_schedule_domain(nvme_scan_ns_async, &scan_info,
> + &domain);
> while (++prev < nsid)
> nvme_ns_remove_by_nsid(ctrl, prev);
> }
> + async_synchronize_full_domain(&domain);
Let me see if I get this right ...
You allocate 'scan_info' on the stack, so every call to
'async_schedule_domain()' in the loop will be using the same
scan_info context, right?
So each instance of nvme_scan_ns_async() will be using
whichever value is in 'next_idx', right?
Effectively making 'nvme_scan_ns_async()' completely free-floating,
spawning 'nr_entry' instances, and letting each instance pick whichever
nsid is (at the time of execution) the next one.
If that's the case then I would welcome some comments in the code, as
this is somewhat non-obvious. And it also spells out clearly why the
atomic 'next_idx' value is absolutely crucial to that mechanism, and
we don't want anyone getting wrong ideas by 'optimizing' that away.
Otherwise:
Reviewed-by: Hannes Reinecke <hare@...e.de>
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
hare@...e.de +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
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