On Tue, May 16, 2017 at 1:54 PM, Adrian Hunter <[email protected]> wrote: > On 10/05/17 11:24, Linus Walleij wrote: >> The mmc_queue_req is a per-request state container the MMC core uses >> to carry bounce buffers, pointers to asynchronous requests and so on. >> Currently allocated as a static array of objects, then as a request >> comes in, a mmc_queue_req is assigned to it, and used during the >> lifetime of the request. >> >> This is backwards compared to how other block layer drivers work: >> they usally let the block core provide a per-request struct that get >> allocated right beind the struct request, and which can be obtained >> using the blk_mq_rq_to_pdu() helper. (The _mq_ infix in this function >> name is misleading: it is used by both the old and the MQ block >> layer.) >> >> The per-request struct gets allocated to the size stored in the queue >> variable .cmd_size initialized using the .init_rq_fn() and >> cleaned up using .exit_rq_fn(). >> >> The block layer code makes the MMC core rely on this mechanism to >> allocate the per-request mmc_queue_req state container. >> >> Doing this make a lot of complicated queue handling go away. > > Isn't that at the expense of increased memory allocation. > > Have you compared the number of allocations? It looks to me like the block > layer allocates a minimum of 4 requests in the memory pool which will > increase if there are more in the I/O scheduler, plus 1 for flush. There > are often 4 queues per eMMC (2x boot,RPMB and main area), so that is 20 > requests minimum, up from 2 allocations previously. For someone using 64K > bounce buffers, you have increased memory allocation by at least 18x64 = > 1152k. However the I/O scheduler could allocate a lot more.
That is not a realistic example. As pointed out in patch #1, bounce buffers are used on old systems which have max_segs == 1. No modern hardware has that, they all have multiple segments-capable host controllers and often also DMA engines. Old systems with max_segs == 1 also have: - One SD or MMC slot - No eMMC (because it was not yet invented in those times) - So no RPMB or Boot partitions, just main area If you can point me to a system that has max_segs == 1 and an eMMC mounted, I can look into it and ask the driver maintainers to check if it disturbs them, but I think those simply do not exist. >> Doing this refactoring is necessary to move the ioctl() operations >> into custom block layer requests tagged with REQ_OP_DRV_[IN|OUT] > > Obviously you could create a per-request data structure with only the > reference to the IOCTL data, and without putting all the memory allocations > there as well. Not easily, and this is the way all IDE, ATA, SCSI disks etc are doing this so why would be try to be different and maintain a lot of deviant code. The allocation of extra data is done by the block layer when issueing blk_get_request() so trying to keep the old mechanism of a list of struct mmc_queue_req and trying to pair these with incoming requests inevitably means a lot of extra work, possibly deepening that list or creating out-of-list extra entries and whatnot. It's better to do what everyone else does and let the core do this allocation of extra data (tag) instead. Yours, Linus Walleij
