Re: [PATCH v2] nvme_core: scan namespaces asynchronously

[stuart hayes <stuart.w.hayes@gmail.com>]

On 1/22/2024 3:13 AM, Sagi Grimberg wrote:
> 
> 
> On 1/18/24 23:03, 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.
>>
>> The time it took for all namespaces to show up after connecting (via TCP)
>> to a controller with 1002 namespaces was measured:
>>
>> network latency   without patch   with patch
>>       0                 6s            1s
>>      50ms             210s           10s
>>     100ms             417s           18s
>>
> 
> Impressive speedup. Not a very common use-case though...
> 
>> Signed-off-by: Stuart Hayes <stuart.w.hayes@gmail.com>
>>
>> -- 
>> V2: remove module param to enable/disable async scanning
>>      add scan time measurements to commit message
>>
>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>> index 0af612387083..069350f85b83 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>
>> @@ -3812,12 +3813,38 @@ static void nvme_validate_ns(struct nvme_ns *ns, struct nvme_ns_info *info)
>>           nvme_ns_remove(ns);
>>   }
>> -static void nvme_scan_ns(struct nvme_ctrl *ctrl, unsigned nsid)
>> +/*
>> + * struct nvme_scan_state - keeps track of controller & NSIDs to scan
>> + * @ctrl:    Controller on which namespaces are being scanned
>> + * @count:    Next NSID to scan (for sequential scan), or
>> + *        Index of next NSID to scan in ns_list (for list scan)
>> + * @ns_list:    pointer to list of NSIDs to scan (NULL if sequential scan)
>> + */
>> +struct nvme_scan_state {
>> +    struct nvme_ctrl *ctrl;
>> +    atomic_t count;
>> +    __le32 *ns_list;
>> +};
>> +
>> +static void nvme_scan_ns(void *data, async_cookie_t cookie)
> 
> I think its better to call it nvme_scan_ns_async to indicate what
> it is.
> 
>>   {
>> -    struct nvme_ns_info info = { .nsid = nsid };
>> +    struct nvme_ns_info info = {};
>> +    struct nvme_scan_state *scan_state;
>> +    struct nvme_ctrl *ctrl;
>> +    u32 nsid;
>>       struct nvme_ns *ns;
>>       int ret;
>> +    scan_state = data;
>> +    ctrl = scan_state->ctrl;
> 
> I think these assignments can be done on the declaration.
> 
>> +    nsid = (u32)atomic_fetch_add(1, &scan_state->count);
>> +    /*
>> +     * get NSID from list (if scanning from a list, not sequentially)
>> +     */
>> +    if (scan_state->ns_list)
>> +        nsid = le32_to_cpu(scan_state->ns_list[nsid]);
>> +
> 
> This is awkward. ns_list passed in optionally.
> How about we limit this change to only operate on nvme_scan_ns_list?
> If the controller is old or quirked to support only a sequential scan
> it does not benefit from a parallel scan. I doubt that these controllers
> are likely to expose a large number of namespaces anyways.
> 
>> +    info.nsid = nsid;
>>       if (nvme_identify_ns_descs(ctrl, &info))
>>           return;
>> @@ -3881,11 +3908,15 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
>>       __le32 *ns_list;
>>       u32 prev = 0;
>>       int ret = 0, i;
>> +    ASYNC_DOMAIN(domain);
>> +    struct nvme_scan_state scan_state;
>>       ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
>>       if (!ns_list)
>>           return -ENOMEM;
>> +    scan_state.ctrl = ctrl;
>> +    scan_state.ns_list = ns_list;
> 
> Is there a need to have a local ns_list variable here?
> 
>>       for (;;) {
>>           struct nvme_command cmd = {
>>               .identify.opcode    = nvme_admin_identify,
>> @@ -3901,19 +3932,25 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
>>               goto free;
>>           }
>> +        /*
>> +         * scan list starting at list offset 0
>> +         */
>> +        atomic_set(&scan_state.count, 0);
>>           for (i = 0; i < nr_entries; i++) {
>>               u32 nsid = le32_to_cpu(ns_list[i]);
>>               if (!nsid)    /* end of the list? */
>>                   goto out;
>> -            nvme_scan_ns(ctrl, nsid);
>> +            async_schedule_domain(nvme_scan_ns, &scan_state, &domain);
>>               while (++prev < nsid)
>>                   nvme_ns_remove_by_nsid(ctrl, prev);
>>           }
>> +        async_synchronize_full_domain(&domain);
>>       }
>>    out:
>>       nvme_remove_invalid_namespaces(ctrl, prev);
> 
> Is it a good idea to remove the invalid namespaces before synchronizing
> the async scans?
> 
>>    free:
>> +    async_synchronize_full_domain(&domain);
>>       kfree(ns_list);
>>       return ret;
>>   }
>> @@ -3922,14 +3959,23 @@ static void nvme_scan_ns_sequential(struct nvme_ctrl *ctrl)
>>   {
>>       struct nvme_id_ctrl *id;
>>       u32 nn, i;
>> +    ASYNC_DOMAIN(domain);
>> +    struct nvme_scan_state scan_state;
>>       if (nvme_identify_ctrl(ctrl, &id))
>>           return;
>>       nn = le32_to_cpu(id->nn);
>>       kfree(id);
>> +    scan_state.ctrl = ctrl;
>> +    /*
>> +     * scan sequentially starting at NSID 1
>> +     */
>> +    atomic_set(&scan_state.count, 1);
>> +    scan_state.ns_list = NULL;
>>       for (i = 1; i <= nn; i++)
>> -        nvme_scan_ns(ctrl, i);
>> +        async_schedule_domain(nvme_scan_ns, &scan_state, &domain);
>> +    async_synchronize_full_domain(&domain);
>>       nvme_remove_invalid_namespaces(ctrl, nn);
>>   }
> 
> I think we need a blktest for this. ns scanning has been notorious when
> running simultaneously with controller reset/reconnect/remove
> sequences... Ideally a test with a larger number of namespaces to
> exercise the code.
> 
> Also, make sure that blktest suite does not complain about anything
> else.

Thank you for the feedback on the patch, I agree with it.

I'm not sure how to implement a blktest suite for this, though.  I can look into it.