On 10/29/21 17:57, Darrick J. Wong wrote:
On Fri, Oct 29, 2021 at 12:46:14PM +0100, Pavel Begunkov wrote:
On 10/28/21 23:59, Dave Chinner wrote:
[...]
Well, my point is doing recovery from bit errors is by definition not
the fast path. Which is why I'd rather keep it away from the pmem
read/write fast path, which also happens to be the (much more important)
non-pmem read/write path.
The trouble is, we really /do/ want to be able to (re)write the failed
area, and we probably want to try to read whatever we can. Those are
reads and writes, not {pre,f}allocation activities. This is where Dave
and I arrived at a month ago.
Unless you'd be ok with a second IO path for recovery where we're
allowed to be slow? That would probably have the same user interface
flag, just a different path into the pmem driver.
I just don't see how 4 single line branches to propage RWF_RECOVERY
down to the hardware is in any way an imposition on the fast path.
It's no different for passing RWF_HIPRI down to the hardware *in the
fast path* so that the IO runs the hardware in polling mode because
it's faster for some hardware.
Not particularly about this flag, but it is expensive. Surely looks
cheap when it's just one feature, but there are dozens of them with
limited applicability, default config kernels are already sluggish
when it comes to really fast devices and it's not getting better.
Also, pretty often every of them will add a bunch of extra checks
to fix something of whatever it would be.
So we can't have data recovery because moving fast the only goal?
That's not what was said and you missed the point, which was in
the rest of the message.
That's so meta.
--D
So let's add a bit of pragmatism to the picture, if there is just one
user of a feature but it adds overhead for millions of machines that
won't ever use it, it's expensive.
This one doesn't spill yet into paths I care about, but in general
it'd be great if we start thinking more about such stuff instead of
throwing yet another if into the path, e.g. by shifting the overhead
from linear to a constant for cases that don't use it, for instance
with callbacks or bit masks.
IOWs, saying that we shouldn't implement RWF_RECOVERY because it
adds a handful of branches the fast path is like saying that we
shouldn't implement RWF_HIPRI because it slows down the fast path
for non-polled IO....
Just factor the actual recovery operations out into a separate
function like:
--
Pavel Begunkov
--
Pavel Begunkov
