Re: [RFC PATCH 4/3] block: skip elevator initialization for flush requests

Tejun Heo
2011-02-01T18:52:33Z

Hello,

On Tue, Feb 01, 2011 at 12:38:46PM -0500, Mike Snitzer wrote:
> > I thought about doing it this way but I think we're burying the
> > REQ_FLUSH|REQ_FUA test logic too deep. get_request() shouldn't
> > "magically" know not to allocate elevator data.
>
> There is already a considerable amount of REQ_FLUSH|REQ_FUA special
> casing magic sprinkled though-out the block layer. Why is this
> get_request() change the case that goes too far?

After the reimplementation, FLUSH implementation seems to be pretty
well isolated. Also, having REQ_FLUSH logic in the issue and
completion paths is logical and preventing them from leaking to other
places sounds like a good idea.

> > The decision should
> > be made higher in the stack and passed down to get_request(). e.g. if
> > REQ_SORTED is set in @rw, elevator data is allocated; otherwise, not.
>
> Considering REQ_SORTED is set in elv_insert(), well after get_request()
> is called, I'm not seeing what you're suggesting.

I was suggesting using REQ_SORTED in @rw parameter to indicate "this
request may be sorted and thus needs elevator data allocation".

> Anyway, I agree that ideally we'd have a mechanism to explicitly
> short-circuit elevator initialization. But doing so in a meaningful way
> would likely require a fair amount of refactoring of get_request* and
> its callers. I'll come back to this and have another look but my gut is
> this interface churn wouldn't _really_ help -- all things considered.

I don't know. I agree that it's not a critical issue but, to me,
subjectively of course, it feels a bit too subtle. The sharing of
fields using unions is already subtle enough. I with that at least
the allocation switching would be obvious and explicit. The combined
subtleties scare me.

Thank you.

--
tejun