Messages in this thread | | | Date | Sun, 21 Nov 2010 21:16:06 +0100 | From | Richard Kralovic <> | Subject | Re: CFQ and dm-crypt |
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>> On 11/03/10 04:23, Jeff Moyer wrote: >>>>> CFQ io scheduler relies on using task_struct current to determine which >>>>> process makes the io request. On the other hand, some dm modules (such >>>>> as dm-crypt) use separate threads for doing io. As CFQ sees only these >>>>> threads, it provides a very poor performance in such a case. >>>>> >>>>> IMHO the correct solution for this would be to store, for every io >>>>> request, the process that initiated it (and preserve this information >>>>> while the request is processed by device mapper). Would that be feasible? >>> Sure. Try the attached patch (still an rfc) and let us know how it >>> goes. In my environment, it speed up multiple concurrent buffered >>> readers. I wasn't able to do a full analysis via blktrace as 2.6.37-rc1 >>> seems to have broken blktrace support on my system. >> >> Thanks for the patch. Unfortunately, I got a kernel panic quite soon >> after booting the patched kernel. I was not able to reproduce the >> panic in a virtual machine, so I had to manually note the backtrace, >> thus I apologize that it's incomplete: > > Hi, Richard, > > I have another patch for you to try. This one holds up pretty well in > my testing using a dm-crypt target. Without the patch, I see no > priority separation:
Hello,
I am sorry for a late reply (it took me some time to try the patch on my main machine). Thank you for your work; unfortunately, I'm still getting a panic at early boot stages: The BUG_ON assert in cfq-iosched.c:cic_free_func failed, with a call trace as follows:
<IRQ> cfq_free_io_context put_io_context cfq_put_request elv_put_request blk_finish_request ...
I applied your patch at Linus's git tree (commit b86db4744230c94e480de56f1b7f31117edbf193), it applied cleanly.
I tried a little to find the source of the problem, but without success so far. However, I think that calling put_io_context in bio_endio only may introduce memory leaks under certain situations (although this is probably not related to the panic I get):
In blk-core.c:blk_rq_prep_clone, __bio_clone allocates the io_context, but if subsequent allocations fail, bio_free in free_and_out does not free it.
Similarly, in dm-crypt.c:crypt_alloc_buffer, bio_put does not free the io_context.
Or have I misunderstood something?
Greets Richard
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