linux-kernel - Re: [PATCH] hpsa: SCSI driver for HP Smart Array controllers

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Date:	Mon, 02 Mar 2009 12:20:42 -0600
From:	Mike Christie <michaelc@...wisc.edu>
To:	Grant Grundler <grundler@...gle.com>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	mike.miller@...com, jens.axboe@...cle.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, hare@...ell.com, iss_storagedev@...com,
	iss.sbteam@...com
Subject: Re: [PATCH] hpsa: SCSI driver for HP Smart Array controllers

Grant Grundler wrote:
> On Sun, Mar 1, 2009 at 10:32 PM, FUJITA Tomonori
> <fujita.tomonori@....ntt.co.jp> wrote:
> ...
>>> +/*
>>> + * For operations that cannot sleep, a command block is allocated at init,
>>> + * and managed by cmd_alloc() and cmd_free() using a simple bitmap to track
>>> + * which ones are free or in use.  Lock must be held when calling this.
>>> + * cmd_free() is the complement.
>>> + */
>>> +static struct CommandList_struct *cmd_alloc(struct ctlr_info *h)
>>> +{
>>> +     struct CommandList_struct *c;
>>> +     int i;
>>> +     union u64bit temp64;
>>> +     dma_addr_t cmd_dma_handle, err_dma_handle;
>>> +
>>> +     do {
>>> +             i = find_first_zero_bit(h->cmd_pool_bits, h->nr_cmds);
>>> +             if (i == h->nr_cmds)
>>> +                     return NULL;
>>> +     } while (test_and_set_bit
>>> +              (i & (BITS_PER_LONG - 1),
>>> +               h->cmd_pool_bits + (i / BITS_PER_LONG)) != 0);
>> Using bitmap to manage free commands looks too complicated a bit to
>> me. Can we just use lists for command management?
> 
> Bit maps are generally more efficient than lists since we touch less data.
> For both search and moving elements from free<->busy lists. This probably
> won't matter if we are talking less than 10K IOPS. And willy demonstrated
> other layers have pretty high overhead (block, libata and SCSI midlayer)
> at high transaction rates.
> 

If it was just needing this for the queuecommand path it would be 
simple. For the queuecommand path we could just use the scsi host 
tagging code for the index. You do not need a lock in the queuecommand 
path for getting a index and command, and you do not need to duplicate 
the tag/index allocation code in the block/scsi code

A problem with the host tagging is what to do if you need a tag/index 
for a internal command. In the slow path like the device reset and cache 
flush case you could use a list or preallocated command or whatever 
other drivers are using that makes you happy.

Or for the reset/shutdown/internal path could we come up with a 
extension to the existing API. Maybe just add some wrapper around some 
of blk_queue_start_tag that takes a the bqt (the bqt would come from the 
host wide one) and allocates the tag (need a something similar for the 
release side).
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