Re: [PATCH] block/mq-deadline: Speed up the dispatch of low-priority requests

[Damien Le Moal]

On Thu, 2021-08-26 at 22:40 +0800, Zhen Lei wrote:
> dd_queued() traverses the percpu variable for summation. The more cores,
> the higher the performance overhead. I currently have a 128-core board and
> this function takes 2.5 us. If the number of high-priority requests is
> small and the number of low- and medium-priority requests is large, the
> performance impact is significant.
> 
> Let's maintain a non-percpu member variable 'nr_queued', which is
> incremented by 1 immediately following "inserted++" and decremented by 1
> immediately following "completed++". Because both the judgment dd_queued()
> in dd_dispatch_request() and operation "inserted++" in dd_insert_request()
> are protected by dd->lock, lock protection needs to be added only in
> dd_finish_request(), which is unlikely to cause significant performance
> side effects.
> 
> Tested on my 128-core board with two ssd disks.
> fio bs=4k rw=read iodepth=128 cpus_allowed=0-95 <others>
> Before:
> [183K/0/0 iops]
> [172K/0/0 iops]
> 
> After:
> [258K/0/0 iops]
> [258K/0/0 iops]
> 
> Fixes: fb926032b320 ("block/mq-deadline: Prioritize high-priority requests")
> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> ---
>  block/mq-deadline.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index a09761cbdf12e58..d8f6aa12de80049 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -79,6 +79,7 @@ struct dd_per_prio {
>  	struct list_head fifo_list[DD_DIR_COUNT];
>  	/* Next request in FIFO order. Read, write or both are NULL. */
>  	struct request *next_rq[DD_DIR_COUNT];
> +	unsigned int nr_queued;
>  };
>  
>  struct deadline_data {
> @@ -277,9 +278,9 @@ deadline_move_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
>  }
>  
>  /* Number of requests queued for a given priority level. */
> -static u32 dd_queued(struct deadline_data *dd, enum dd_prio prio)
> +static __always_inline u32 dd_queued(struct deadline_data *dd, enum dd_prio prio)
>  {
> -	return dd_sum(dd, inserted, prio) - dd_sum(dd, completed, prio);
> +	return dd->per_prio[prio].nr_queued;
>  }
>  
>  /*
> @@ -711,6 +712,8 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  
>  	prio = ioprio_class_to_prio[ioprio_class];
>  	dd_count(dd, inserted, prio);
> +	per_prio = &dd->per_prio[prio];
> +	per_prio->nr_queued++;
>  
>  	if (blk_mq_sched_try_insert_merge(q, rq, &free)) {
>  		blk_mq_free_requests(&free);
> @@ -719,7 +722,6 @@ static void dd_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
>  
>  	trace_block_rq_insert(rq);
>  
> -	per_prio = &dd->per_prio[prio];
>  	if (at_head) {
>  		list_add(&rq->queuelist, &per_prio->dispatch);
>  	} else {
> @@ -790,12 +792,14 @@ static void dd_finish_request(struct request *rq)
>  	const u8 ioprio_class = dd_rq_ioclass(rq);
>  	const enum dd_prio prio = ioprio_class_to_prio[ioprio_class];
>  	struct dd_per_prio *per_prio = &dd->per_prio[prio];
> +	unsigned long flags;
>  
>  	dd_count(dd, completed, prio);
> +	spin_lock_irqsave(&dd->lock, flags);
> +	per_prio->nr_queued--;
> +	spin_unlock_irqrestore(&dd->lock, flags);

dd->lock is not taken with irqsave everywhere else. This leads to hard lockups
which I hit right away on boot. To avoid this, we need a spin_lock_irqsave()
everywhere.

Of note is that without this patch, testing on nullblk with Bart's script on
5.14.0-rc7, I get this splat:

 [  198.726920] watchdog: BUG: soft lockup - CPU#20 stuck for 26s!
[kworker/20:1H:260]
[  198.734550] Modules linked in: null_blk rpcsec_gss_krb5 auth_rpcgss nfsv4
dns_resolver nfs lockd grace fscache netfs nft_fib_inet nft_fib_ipv4
nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject
nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set
nf_tables libcrc32c nfnetlink sunrpc vfat fat iTCO_wdt iTCO_vendor_support
ipmi_ssif x86_pkg_temp_thermal acpi_ipmi coretemp ipmi_si ioatdma i2c_i801 bfq
i2c_smbus lpc_ich intel_pch_thermal dca ipmi_devintf ipmi_msghandler
acpi_power_meter fuse ip_tables sd_mod ast i2c_algo_bit drm_vram_helper
drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops drm_ttm_helper ttm
drm i40e crct10dif_pclmul mpt3sas crc32_pclmul ahci ghash_clmulni_intel libahci
libata raid_class scsi_transport_sas pkcs8_key_parser
[  198.805375] irq event stamp: 25378690
[  198.809063] hardirqs last  enabled at (25378689): [<ffffffff81149959>]
ktime_get+0x109/0x120
[  198.817545] hardirqs last disabled at (25378690): [<ffffffff8190519b>]
sysvec_apic_timer_interrupt+0xb/0x90
[  198.827327] softirqs last  enabled at (25337302): [<ffffffff810b331f>]
__irq_exit_rcu+0xbf/0xe0
[  198.836066] softirqs last disabled at (25337297): [<ffffffff810b331f>]
__irq_exit_rcu+0xbf/0xe0
[  198.844802] CPU: 20 PID: 260 Comm: kworker/20:1H Not tainted 5.14.0-rc7+
#1324
[  198.852059] Hardware name: Supermicro Super Server/X11DPL-i, BIOS 3.3
02/21/2020
[  198.859487] Workqueue: kblockd blk_mq_run_work_fn
[  198.864222] RIP: 0010:__list_add_valid+0x33/0x40
[  198.868868] Code: f2 0f 85 ec 44 44 00 4c 8b 0a 4d 39 c1 0f 85 08 45 44 00
48 39 d7 0f 84 e8 44 44 00 4c 39 cf 0f 84 df 44 44 00 b8 01 00 00 00 <c3> 66 66
2e 0f 1f 84 00 00 00 00 00 90 48 8b 17 4c 8b 47 08 48 b8
[  198.887712] RSP: 0018:ffff8883f1337d68 EFLAGS: 00000206
[  198.892963] RAX: 0000000000000001 RBX: ffff88857dae0840 RCX:
0000000000000000
[  198.900132] RDX: ffff8885387e2bc8 RSI: ffff8885387e2bc8 RDI:
ffff8885387e2d48
[  198.907300] RBP: ffff8883f1337d90 R08: ffff8883f1337d90 R09:
ffff8883f1337d90
[  198.914467] R10: 0000000000000020 R11: 0000000000000001 R12:
000000000000000a
[  198.921632] R13: ffff88857dae0800 R14: ffff8885bd3f3400 R15:
ffff8885bd276200
[  198.928801] FS:  0000000000000000(0000) GS:ffff888860100000(0000)
knlGS:0000000000000000
[  198.936929] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  198.942703] CR2: 0000000002204440 CR3: 0000000107322004 CR4:
00000000007706e0
[  198.949871] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[  198.957036] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
0000000000000400
[  198.964203] PKRU: 55555554
[  198.966933] Call Trace:
[  198.969401]  __blk_mq_do_dispatch_sched+0x234/0x2f0
[  198.974314]  __blk_mq_sched_dispatch_requests+0xf4/0x140
[  198.979662]  blk_mq_sched_dispatch_requests+0x30/0x60
[  198.984744]  __blk_mq_run_hw_queue+0x49/0x90
[  198.989041]  process_one_work+0x26c/0x570
[  198.993083]  worker_thread+0x55/0x3c0
[  198.996776]  ? process_one_work+0x570/0x570
[  199.000993]  kthread+0x140/0x160
[  199.004243]  ? set_kthread_struct+0x40/0x40
[  199.008452]  ret_from_fork+0x1f/0x30




>  
>  	if (blk_queue_is_zoned(q)) {
> -		unsigned long flags;
> -
>  		spin_lock_irqsave(&dd->zone_lock, flags);
>  		blk_req_zone_write_unlock(rq);
>  		if (!list_empty(&per_prio->fifo_list[DD_WRITE]))

-- 
Damien Le Moal
Western Digital Research