Re: [RFC] add sysfs to dynamically control blk request tagmaintenance

[Arjan van de Ven]

> The request itself has been
> touched by scsi_end_request() already, so unless the layout is really
> bad we shouldn't need to fetch a lot of cache lines there. That leaves
> __test_and_clear_bit(), I guess that must be it.

__test_and_clear_bit() is cheap; it's a single non-locked instruction.
Arguably it should be converted into a C variant so that the compiler
can pick the best code sequence (I'm not so sure the asm we picked for
this is optimal for all processors, I bet it'll be microcoded) but I'd
be surprised if it'd be more than a handful of cycles no matter what.


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