Re: [PATCH] mmc: core: avoid blocking CPU for long BKOPS

[Ulf Hansson <ulf.hansson@linaro.org>]

On Sat, 14 Dec 2019 at 18:59, Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote:
>
> Since the timeout for a command might be 10 minutes (fallback), don't
> eat all CPU power spinning for completion (triggering lockup detector).
> Implement delay with exponential backoff and a one second limit.
>
> [158480.011769] watchdog: BUG: soft lockup - CPU#3 stuck for 23s! [fsck.f2fs:962]
> [158480.014911] Modules linked in: brcmfmac brcmutil cfg80211 mmc_block sdhci_tegra cqhci sdhci_pltfm sdhci pwrseq_simple pwrseq_emmc mmc_core
> [158480.018291] CPU: 3 PID: 962 Comm: fsck.f2fs Not tainted 5.5.0-rc1-next-20191209mq-00173-g716e74177313-dirty #95
> [158480.021479] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> [158480.024934] PC is at sdhci_card_busy+0x28/0x44 [sdhci]
> [158480.028858] LR is at __mmc_switch+0x180/0x330 [mmc_core]
> [158480.032449] pc : [<af03b118>]    lr : [<af009fcc>]    psr: 60000013
> [158480.036004] sp : e79c9b38  ip : ee01dfd0  fp : 00f23d31
> [158480.039339] r10: 00000000  r9 : 0000e020  r8 : b0c04900
> [158480.042939] r7 : e763e000  r6 : 00000000  r5 : ee189000  r4 : ee189000
> [158480.044909] r3 : f002b600  r2 : af03b0f0  r1 : ee01db00  r0 : 1fe70000
> [158480.046857] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
> [158480.048870] Control: 10c5387d  Table: b76ac04a  DAC: 00000055
> [158480.050807] CPU: 3 PID: 962 Comm: fsck.f2fs Not tainted 5.5.0-rc1-next-20191209mq-00173-g716e74177313-dirty #95
> [158480.052913] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> [...]
> [158480.098604] [<b0101ab8>] (__irq_svc) from [<af03b118>] (sdhci_card_busy+0x28/0x44 [sdhci])
> [158480.101406] [<af03b118>] (sdhci_card_busy [sdhci]) from [<af009fcc>] (__mmc_switch+0x180/0x330 [mmc_core])
> [158480.104348] [<af009fcc>] (__mmc_switch [mmc_core]) from [<af00a3bc>] (mmc_run_bkops+0xdc/0x114 [mmc_core])
> [158480.107209] [<af00a3bc>] (mmc_run_bkops [mmc_core]) from [<af067d88>] (mmc_blk_mq_complete_prev_req.part.4+0x8c/0x21c [mmc_block])
> [158480.110043] [<af067d88>] (mmc_blk_mq_complete_prev_req.part.4 [mmc_block]) from [<af067f94>] (mmc_blk_rw_wait+0x7c/0x11c [mmc_block])
> [158480.112914] [<af067f94>] (mmc_blk_rw_wait [mmc_block]) from [<af068c34>] (mmc_blk_mq_issue_rq+0x318/0x898 [mmc_block])
> [158480.115733] [<af068c34>] (mmc_blk_mq_issue_rq [mmc_block]) from [<af069608>] (mmc_mq_queue_rq+0x124/0x244 [mmc_block])
> [158480.118466] [<af069608>] (mmc_mq_queue_rq [mmc_block]) from [<b04560b0>] (__blk_mq_try_issue_directly+0x118/0x1a0)
> [158480.121200] [<b04560b0>] (__blk_mq_try_issue_directly) from [<b04571c0>] (blk_mq_request_issue_directly+0x40/0x5c)
> [158480.123980] [<b04571c0>] (blk_mq_request_issue_directly) from [<b0457228>] (blk_mq_try_issue_list_directly+0x4c/0xc0)
> [158480.126780] [<b0457228>] (blk_mq_try_issue_list_directly) from [<b045bc70>] (blk_mq_sched_insert_requests+0x1b0/0x234)
> [158480.129601] [<b045bc70>] (blk_mq_sched_insert_requests) from [<b04570a8>] (blk_mq_flush_plug_list+0x318/0x3f0)
> [158480.132446] [<b04570a8>] (blk_mq_flush_plug_list) from [<b044bcb0>] (blk_flush_plug_list+0xc0/0xc8)
> [158480.135292] [<b044bcb0>] (blk_flush_plug_list) from [<b044bcec>] (blk_finish_plug+0x34/0x4c)
> [158480.138113] [<b044bcec>] (blk_finish_plug) from [<b024104c>] (read_pages+0x60/0x150)
> [158480.140988] [<b024104c>] (read_pages) from [<b0241338>] (__do_page_cache_readahead+0x1fc/0x20c)
> [158480.143893] [<b0241338>] (__do_page_cache_readahead) from [<b02361f8>] (generic_file_read_iter+0x9ac/0xd28)
> [158480.146818] [<b02361f8>] (generic_file_read_iter) from [<b02b6920>] (__vfs_read+0x128/0x1a8)
> [158480.149695] [<b02b6920>] (__vfs_read) from [<b02b6a34>] (vfs_read+0x94/0x11c)
> [158480.152638] [<b02b6a34>] (vfs_read) from [<b02b6d34>] (ksys_read+0x50/0xbc)
> [158480.155601] [<b02b6d34>] (ksys_read) from [<b0101000>] (ret_fast_syscall+0x0/0x50)
> [158480.158490] Exception stack(0xe79c9fa8 to 0xe79c9ff0)
> [158480.161430] 9fa0:                   00000003 01bec3b8 00000003 01bec3b8 00001000 00000000
> [158480.164422] 9fc0: 00000003 01bec3b8 00000000 00000003 01bec3b8 01bdf178 a6fc3f70 00000000
> [158480.167385] 9fe0: a6f95074 aecadaf8 a6f82c58 a6ef2284

Thanks for looking into this, it's been on my TODO list for quite a while.

Just so I understand correctly, the "soft lock up", gets released when
mmc_run_bkops() completes after 23s? Or are you triggering this via
some other "manual hacks"?

>
> Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> ---
>  drivers/mmc/core/mmc_ops.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/mmc/core/mmc_ops.c b/drivers/mmc/core/mmc_ops.c
> index 09113b9ad679..e3005e5b9509 100644
> --- a/drivers/mmc/core/mmc_ops.c
> +++ b/drivers/mmc/core/mmc_ops.c
> @@ -20,6 +20,7 @@
>  #include "mmc_ops.h"
>
>  #define MMC_OPS_TIMEOUT_MS     (10 * 60 * 1000) /* 10 minute timeout */
> +#define MMC_MAX_POLL_MS                (1000) /* 1 second */
>
>  static const u8 tuning_blk_pattern_4bit[] = {
>         0xff, 0x0f, 0xff, 0x00, 0xff, 0xcc, 0xc3, 0xcc,
> @@ -454,6 +455,7 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
>         struct mmc_host *host = card->host;
>         int err;
>         unsigned long timeout;
> +       unsigned int wait_ms;
>         u32 status = 0;
>         bool expired = false;
>         bool busy = false;
> @@ -473,6 +475,7 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
>         }
>
>         timeout = jiffies + msecs_to_jiffies(timeout_ms) + 1;
> +       wait_ms = 1;
>         do {
>                 /*
>                  * Due to the possibility of being preempted while polling,
> @@ -482,6 +485,11 @@ static int mmc_poll_for_busy(struct mmc_card *card, unsigned int timeout_ms,
>
>                 if (host->ops->card_busy) {
>                         busy = host->ops->card_busy(host);
> +                       if (busy && !expired) {
> +                               mmc_delay(wait_ms);
> +                               if (wait_ms < MMC_MAX_POLL_MS)
> +                                       wait_ms *= 2;

I am not sure a pure exponential method is the best solution, but
let's discuss this.

For example, let's assume timeout_ms is 250ms. The actual time it
takes for the card to stop signalling busy (in the ideal case), is
50ms.

How far off would we then be from 50ms by using the above solution?

Another example is when timeout_ms is set to 60s, but we should hit as
close as possible to 5s. How far off would the solution above be from
that?


> +                       }
>                 } else {
>                         err = mmc_send_status(card, &status);
>                         if (retry_crc_err && err == -EILSEQ) {
> --
> 2.20.1
>

Kind regards
Uffe