Re: [PATCH] writeback: remove unnecessary wait in throttle_vm_writeout()

[Ingo Molnar <mingo@elte.hu>]

* Andrew Morton <akpm@linux-foundation.org> wrote:

> This is a pretty major bugfix.
> 
> GFP_NOIO and GFP_NOFS callers should have been spending really large 
> amounts of time stuck in that sleep.
> 
> I wonder why nobody noticed this happening.  Either a) it turns out 
> that kswapd is doing a good job and such callers don't do direct 
> reclaim much or b) nobody is doing any in-depth kernel 
> instrumentation.

[ Oh, it's Friday already, so soapbox time i guess. The easily offended 
  please skip this mail ;-) ]

People _have_ noticed, and we often ignored them. I can see four 
fundamental, structural problems:

1) A certain lack of competitive pressure. An MM is too complex and
   there is no "better Linux MM" to compare against objectively. The
   BSDs are way too different and it's easy to dismiss even objective
   comparisons due to the real complexity of the differences. Heck,
   2.6.9 is "way too different" and we routinely reject bugreports from
   such old kernels and lose vital feedback.

2) There is a wide-spread mentality of "you prove that there is a
   problem" in the MM and elsewhere in the Linux kernel too. While of 
   course objective proof is paramount, we often "hide" behind our 
   self-created complexity of the system (without malice and without 
   realising it!). We've seen that happen in the updatedb discussions 
   and the swap-prefetch discussions. The correct approach would be for 
   the MM folks to be able to tell for just about any workload "this is 
   not our problem", and to have the benefit of the doubt _on the 
   tester's side_. We must not ignore people who tell us that "there is 
   something wrong going on here", just because they are unable to 
   analyze it themselves. Very often where we end up saying "we dont 
   know what's going on here" it's likely _our_ fault. We also must not 
   hide behind "please do these 10 easy steps and 2 kernel recompiles 
   and 10 reboots, only takes half a day, and come back to us once you 
   have the detailed debug data" requests. Instrumentation must be _on 
   by default_ (like SCHED_DEBUG is on by default), which brings us to:

3) Instrumentation and tools. Instrumentation (for example MM delay 
   statistics - like the scheduler delay statistics) give an objective 
   measure to compare kernels against each other. _Smart_ and _easy to 
   use_ and _default enabled_ instrumentation is a must. Not "turn on 
   these 3 zillion kernel options" which no distro enables. Debug 
   tools/scripts that use the instrumentation, that just have to be run 
   and produce meaningful output based on which 90% of the workloads can 
   be analyzed _without having to ask the user to do more_. (See 
   PowerTop as an example, the right kind of instrumentation can do 
   wonders that enables users to help us. We worked hard to lower the 
   cost of /proc/timer_stats so that distros can enable it by default - 
   and now they do enable it by default.)

4) The use of heuristics and the resulting inevitable nondeterminism in 
   the MM. I guess i'm biased about this, doing -rt and CFS, but we've 
   seen that happen with the scheduler: users _love_ determinism. (Users
   dont typically care whether a click on the desktop takes 0.5 seconds 
   or 1.0 second - as long as it's always 0.5 or always 1.0. What they
   do notice is when a click takes 0.5 seconds most of the time but
   occasionally it takes 1.5 seconds - _that_ they report as a 
   regression. They would actually prefer it to take 1.0 seconds all the 
   time. The reason is human psychology: 99% of our daily routine is 
   driven by inconscious brain automatisms. We auto-pilot through most 
   of the day - and that very much covers routine computer/desktop usage
   too. Unpredictable/noisy behavior of the computer forces the human 
   brain back into more consious activity, which is perceived as a 
   negative thing: it's a distraction takes capacity away from 
   _important_ conscious activities ... such as getting real work done 
   on the computer.)

   Heuristics is also an objective problem for the code itself: it 
   introduces artificial coupling of workloads and raises complexity 
   artificially: it makes it very hard to prove the impact of changes 
   (even with good instrumentation) - thus increasing the barrier of 
   entry significantly. (both to external contributors and to existing
   maintainers)

all in one: the barrier of entry to _providing meaningful feedback_ is 
often very high, and thus the barrier of entry of experimental patches 
is too high too. These two factors are a lethal combination that lure us 
into the false perception that everything is fine and that the yelling 
out there is just from clueless whiners who are not willing to help us 
:-/

Yes, MM testing is hard (in fact, good MM instrumentation and tooling is 
_very_ hard), and the MM is in a pretty good shape (otherwise an 
alternative would have shown up already), and today's MM is clearly the 
best ever Linux MM - but still we have to solve these structural 
problems if we want to advance to the next level of quality.

The solution? I think it's not that hard: we should lower the acceptance 
barrier of instrumentation patches massively. (maybe even merge them 
outside the normal merge window, like we merge cleanups) Then we should 
only allow high-rate changes in risky kernel subsystems that improve 
their own instrumentation and tools sufficiently for ordinary users to 
be able to tell whether the changes are an improvement or not. Every 
time there's a major regression that was hard to debug via the existing 
instrumentation, mandate the extension of instrumentation to cover that 
case too.

This all couples the desire of developers to add new code with the 
desire of testers to provide feedback and with the desire of actual 
users to have a proven good system.

	Ingo
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