Re: [PATCH] fix readahead pipeline break caused by block plug

[Vivek Goyal]

On Tue, Jan 31, 2012 at 03:59:40PM +0800, Shaohua Li wrote:
> Herbert Poetzl reported a performance regression since 2.6.39. The test
> is a simple dd read, but with big block size. The reason is:
> 
> T1: ra (A, A+128k), (A+128k, A+256k)
> T2: lock_page for page A, submit the 256k
> T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
> because of plug and there isn't any lock_page till we hit page A+256k
> because all pages from A to A+256k is in memory
> T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
> submitted again.

Why IO is not submitted because of plug? Doesn't task now get scheduled
out causing an unplug? IOW, are we now busy waiting somewhere preventing
unplug?

Thanks
Vivek
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