Documentation: SubmittingPatches: overhaul changelog description

Maintainers often repeat the same feedback on poorly written
changelogs - describe the problem, justify your changes, quantify
optimizations, describe user-visible changes - but our documentation
on writing changelogs doesn't include these things.  Fix that.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmittingPatches

@@ -84,18 +84,42 @@ is another popular alternative.
 
 2) Describe your changes.
 
-Describe the technical detail of the change(s) your patch includes.
-
-Be as specific as possible.  The WORST descriptions possible include
-things like "update driver X", "bug fix for driver X", or "this patch
-includes updates for subsystem X.  Please apply."
+Describe your problem.  Whether your patch is a one-line bug fix or
+5000 lines of a new feature, there must be an underlying problem that
+motivated you to do this work.  Convince the reviewer that there is a
+problem worth fixing and that it makes sense for them to read past the
+first paragraph.
+
+Describe user-visible impact.  Straight up crashes and lockups are
+pretty convincing, but not all bugs are that blatant.  Even if the
+problem was spotted during code review, describe the impact you think
+it can have on users.  Keep in mind that the majority of Linux
+installations run kernels from secondary stable trees or
+vendor/product-specific trees that cherry-pick only specific patches
+from upstream, so include anything that could help route your change
+downstream: provoking circumstances, excerpts from dmesg, crash
+descriptions, performance regressions, latency spikes, lockups, etc.
+
+Quantify optimizations and trade-offs.  If you claim improvements in
+performance, memory consumption, stack footprint, or binary size,
+include numbers that back them up.  But also describe non-obvious
+costs.  Optimizations usually aren't free but trade-offs between CPU,
+memory, and readability; or, when it comes to heuristics, between
+different workloads.  Describe the expected downsides of your
+optimization so that the reviewer can weigh costs against benefits.
+
+Once the problem is established, describe what you are actually doing
+about it in technical detail.  It's important to describe the change
+in plain English for the reviewer to verify that the code is behaving
+as you intend it to.
 
 The maintainer will thank you if you write your patch description in a
 form which can be easily pulled into Linux's source code management
 system, git, as a "commit log".  See #15, below.
 
-If your description starts to get long, that's a sign that you probably
-need to split up your patch.  See #3, next.
+Solve only one problem per patch.  If your description starts to get
+long, that's a sign that you probably need to split up your patch.
+See #3, next.
 
 When you submit or resubmit a patch or patch series, include the
 complete patch description and justification for it.  Don't just