Re: [PATCH V2] lightnvm: pblk: prevent stall due to wb threshold

[Hans Holmberg]

Hi Javier!

How did you test this? I'm trying to add a test case to our testing framework.

This is what i ran in qemu, and I got a hang (with this version of the patch)

nvme lnvm create -d nvme0n1 -t pblk -n pblk0 -f -b 0 -e 0

kernel log: [  116.381799] pblk pblk0: luns:1, lines:280, secs:212736,
buf entries:128

# dd if=/dev/zero of=/dev/pblk0 oflag=direct bs=4k count=1
1+0 records in
1+0 records out
4096 bytes (4.1 kB, 4.0 KiB) copied, 0.000480941 s, 8.5 MB/s
# dd if=/dev/zero of=/dev/pblk0 oflag=direct bs=64k count=1
1+0 records in
1+0 records out
65536 bytes (66 kB, 64 KiB) copied, 0.000477373 s, 137 MB/s
# dd if=/dev/zero of=/dev/pblk0 oflag=direct bs=128k count=1
1+0 records in
1+0 records out
131072 bytes (131 kB, 128 KiB) copied, 0.000548722 s, 239 MB/s
# dd if=/dev/zero of=/dev/pblk0 oflag=direct bs=256k count=1
1+0 records in
1+0 records out
262144 bytes (262 kB, 256 KiB) copied, 0.000718515 s, 365 MB/s
# dd if=/dev/zero of=/dev/pblk0 oflag=direct bs=512k count=1
<HANG>


On Wed, Jan 30, 2019 at 11:28 AM Javier GonzÃlez <javier@xxxxxxxxxxx> wrote:
>
> In order to respect mw_cuinits, pblk's write buffer maintains a
> backpointer to protect data not yet persisted; when writing to the write
> buffer, this backpointer defines a threshold that pblk's rate-limiter
> enforces.
>
> On small PU configurations, the following scenarios might take place: (i)
> the threshold is larger than the write buffer and (ii) the threshold is
> smaller than the write buffer, but larger than the maximun allowed
> split bio - 256KB at this moment (Note that writes are not always
> split - we only do this when we the size of the buffer is smaller
> than the buffer). In both cases, pblk's rate-limiter prevents the I/O to
> be written to the buffer, thus stalling.
>
> This patch fixes the original backpointer implementation by considering
> the threshold both on buffer creation and on the rate-limiters path,
> when bio_split is triggered (case (ii) above).
>
> Fixes: 766c8ceb16fc ("lightnvm: pblk: guarantee that backpointer is respected on writer stall")
> Signed-off-by: Javier GonzÃlez <javier@xxxxxxxxxxx>
> ---
>
>   Changes since V1:
>     - Fix a bad arithmetinc on the rate-limiter max_io calculation (from
>       Hans)
>
>  drivers/lightnvm/pblk-rb.c | 25 +++++++++++++++++++------
>  drivers/lightnvm/pblk-rl.c |  5 ++---
>  drivers/lightnvm/pblk.h    |  2 +-
>  3 files changed, 22 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/lightnvm/pblk-rb.c b/drivers/lightnvm/pblk-rb.c
> index d4ca8c64ee0f..a6133b50ed9c 100644
> --- a/drivers/lightnvm/pblk-rb.c
> +++ b/drivers/lightnvm/pblk-rb.c
> @@ -45,10 +45,23 @@ void pblk_rb_free(struct pblk_rb *rb)
>  /*
>   * pblk_rb_calculate_size -- calculate the size of the write buffer
>   */
> -static unsigned int pblk_rb_calculate_size(unsigned int nr_entries)
> +static unsigned int pblk_rb_calculate_size(unsigned int nr_entries,
> +                                          unsigned int threshold)
>  {
> -       /* Alloc a write buffer that can at least fit 128 entries */
> -       return (1 << max(get_count_order(nr_entries), 7));
> +       unsigned int thr_sz = 1 << (get_count_order(threshold + NVM_MAX_VLBA));
> +       unsigned int max_sz = max(thr_sz, nr_entries);
> +       unsigned int max_io;
> +
> +       /* Alloc a write buffer that can (i) fit at least two split bios
> +        * (considering max I/O size NVM_MAX_VLBA, and (ii) guarantee that the
> +        * threshold will be respected
> +        */
> +       max_io = (1 << max((int)(get_count_order(max_sz)),
> +                               (int)(get_count_order(NVM_MAX_VLBA << 1))));
> +       if ((threshold + NVM_MAX_VLBA) >= max_io)
> +               max_io <<= 1;
> +
> +       return max_io;
>  }
>
>  /*
> @@ -67,12 +80,12 @@ int pblk_rb_init(struct pblk_rb *rb, unsigned int size, unsigned int threshold,
>         unsigned int alloc_order, order, iter;
>         unsigned int nr_entries;
>
> -       nr_entries = pblk_rb_calculate_size(size);
> +       nr_entries = pblk_rb_calculate_size(size, threshold);
>         entries = vzalloc(array_size(nr_entries, sizeof(struct pblk_rb_entry)));
>         if (!entries)
>                 return -ENOMEM;
>
> -       power_size = get_count_order(size);
> +       power_size = get_count_order(nr_entries);
>         power_seg_sz = get_count_order(seg_size);
>
>         down_write(&pblk_rb_lock);
> @@ -149,7 +162,7 @@ int pblk_rb_init(struct pblk_rb *rb, unsigned int size, unsigned int threshold,
>          * Initialize rate-limiter, which controls access to the write buffer
>          * by user and GC I/O
>          */
> -       pblk_rl_init(&pblk->rl, rb->nr_entries);
> +       pblk_rl_init(&pblk->rl, rb->nr_entries, threshold);
>
>         return 0;
>  }
> diff --git a/drivers/lightnvm/pblk-rl.c b/drivers/lightnvm/pblk-rl.c
> index 76116d5f78e4..e9e0af0df165 100644
> --- a/drivers/lightnvm/pblk-rl.c
> +++ b/drivers/lightnvm/pblk-rl.c
> @@ -207,7 +207,7 @@ void pblk_rl_free(struct pblk_rl *rl)
>         del_timer(&rl->u_timer);
>  }
>
> -void pblk_rl_init(struct pblk_rl *rl, int budget)
> +void pblk_rl_init(struct pblk_rl *rl, int budget, int threshold)
>  {
>         struct pblk *pblk = container_of(rl, struct pblk, rl);
>         struct nvm_tgt_dev *dev = pblk->dev;
> @@ -217,7 +217,6 @@ void pblk_rl_init(struct pblk_rl *rl, int budget)
>         int sec_meta, blk_meta;
>         unsigned int rb_windows;
>
> -
>         /* Consider sectors used for metadata */
>         sec_meta = (lm->smeta_sec + lm->emeta_sec[0]) * l_mg->nr_free_lines;
>         blk_meta = DIV_ROUND_UP(sec_meta, geo->clba);
> @@ -234,7 +233,7 @@ void pblk_rl_init(struct pblk_rl *rl, int budget)
>         /* To start with, all buffer is available to user I/O writers */
>         rl->rb_budget = budget;
>         rl->rb_user_max = budget;
> -       rl->rb_max_io = budget >> 1;
> +       rl->rb_max_io = budget - threshold;
>         rl->rb_gc_max = 0;
>         rl->rb_state = PBLK_RL_HIGH;
>
> diff --git a/drivers/lightnvm/pblk.h b/drivers/lightnvm/pblk.h
> index 72ae8755764e..a6386d5acd73 100644
> --- a/drivers/lightnvm/pblk.h
> +++ b/drivers/lightnvm/pblk.h
> @@ -924,7 +924,7 @@ int pblk_gc_sysfs_force(struct pblk *pblk, int force);
>  /*
>   * pblk rate limiter
>   */
> -void pblk_rl_init(struct pblk_rl *rl, int budget);
> +void pblk_rl_init(struct pblk_rl *rl, int budget, int threshold);
>  void pblk_rl_free(struct pblk_rl *rl);
>  void pblk_rl_update_rates(struct pblk_rl *rl);
>  int pblk_rl_high_thrs(struct pblk_rl *rl);
> --
> 2.17.1
>