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SubjectRe: [PATCH 9/11] Panic delay fix
Pavel Machek wrote:
> On Thu 2007-02-08 07:36:12, Rusty Russell wrote:
>> On Wed, 2007-02-07 at 12:35 +0000, Pavel Machek wrote:
>>> Ugh, it sounds like paravirt is more b0rken then I thought. It should
>>> always to the proper delay, then replace those udelays that are not
>>> needed on virtualized hardware with something else.
>>> Just magically defining udelay into nop is broken.
>> We'd have to audit and figure out what udelays are for hardware and
>> which are not, but the evidence is that the vast majority of them are
>> for hardware and not needed for virtualization.
> You did not time to do the full audit, so you just did #define.

Yes, of course. Since 99% of the drivers are completely irrelevant for
paravirt, and 99% of the udelays are in drivers, there isn't much point
to auditing a bunch of code we're not even going to be affected by. The
default case for udelay is it is not needed.

>> Changing udelay to "hardware_udelay" or something all over the kernel
>> would have delayed the paravirt_ops merge by an infinite amount 8)
> And here you claim you could not do the right thing, because people
> would notice you are doing huge search/replace without audit, and
> would stop you. So you simply hidden it from them :-(.

What ludicrousness is this? Hidden what? That the default case for
udelay is that it is not needed?

> Plus... udelay() should just work under virtualization, right? You get
> slightly slower kernel, but still working, so the "full audit" is not
> as hard as you are telling me.

Save the time of doing a useless full audit and making sure we didn't
accidentally redefine or misspell some symbol on a bunch of
architectures we aren't even set up to compile for.

> Just replace udelay() with hardware_udelay() on places that matter in
> your workload...

That's inconsistent. We would be doing 2 SCSI drivers, part of the IDE
code, some i386 arch code, some random places in the kernel... and now
nobody else knows whether to use udelay or hardware_udelay and the code
gets jumbled to the point that it is useless because there is no clear
distinction between the two. It is non-trivial to come up with a list
of source files that we have to actually do this to. One C-file calls a
shared routine in a library, and now you've got a hidden udelay that you
have absolutely no way of detecting. The right thing to do if you want
to do it on a line by line basis is exactly the opposite. Remove udelay
and find out what breaks. Bugs are easier to find and fix than hidden
code. If I were to do it on a line by line basis, I would chose to
replace udelay() with real_time_udelay() for those places that actually
need it.

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