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Date: Thu, 31 Jan 2019 16:33:06 +0000
From: Javier González <javier@...igon.com>
To: Hans Holmberg <hans@...tronix.com>
Cc: Matias Bjorling <mb@...htnvm.io>,
Hans Holmberg <hans.holmberg@...xlabs.com>,
linux-block@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] lightnvm: pblk: prevent stall due to wb threshold
> On 31 Jan 2019, at 11.41, Hans Holmberg <hans@...tronix.com> wrote:
>
> 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
I run several low configurations without problem. Can you share the qemu configuration and version?
I’m on travel until Friday - I’ll come back to you over the weekend.
>
> 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 González <javier@...igon.com> 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 González <javier@...igon.com>
>> ---
>>
>> 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
>>
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