Re: [RFC PATCH v2 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

On Fri, 20 Mar 2026 08:29:39 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:

> Adding Sashiko comment
> (https://lore.kernel.org/https://sashiko.dev/#/patchset/20260319052157.99433-11-sj@xxxxxxxxxx)
> with ': ' line prefix, and my replies in line.
> 
> On Wed, 18 Mar 2026 22:21:53 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:
> 
> > The sysfs.py test commits DAMON parameters, dump the internal DAMON
> > state, and show if the parameters are committed as expected using the
> > dumped state.  While the dumping is ongoing, DAMON is alive.  It can
> > make internal changes including addition and removal of regions.  It can
> > therefore make a race that can result in false test results.  Pause
> > DAMON execution during the state dumping to avoid such races.
> > 
> > Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> > ---
> >  tools/testing/selftests/damon/sysfs.py | 31 ++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
> > index e6d34ba05893f..704729c7a318e 100755
> > --- a/tools/testing/selftests/damon/sysfs.py
> > +++ b/tools/testing/selftests/damon/sysfs.py
> > @@ -193,18 +193,48 @@ def assert_ctx_committed(ctx, dump):
> >      assert_true(dump['pause'] == ctx.pause, 'pause', dump)
> >  
> >  def assert_ctxs_committed(kdamonds):
> > +    ctxs_paused_for_dump = []
> > +    for kd in kdamonds.kdamonds:
> > +        for ctx in kd.contexts:
> > +            if ctx.pause is False:
> > +                ctx.pause = True
> > +                err = kd.commit()
> > +                if err is not None:
> > +                    print('pause fail (%s)' % err)
> > +                    kdamonds.stop()
> > +                    exit(1)
> > +                ctxs_paused_for_dump.append(ctx)
> > +
> >      status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
> >      if err is not None:
> >          print(err)
> >          kdamonds.stop()
> >          exit(1)
> >  
> > +    for kd in kdamonds.kdamonds:
> > +        for ctx in kd.contexts:
> > +            if ctx in ctxs_paused_for_dump:
> > +                ctx.pause = False
> > +                err = kd.commit()
> > +                if err is not None:
> > +                    print('resume fail (%s)' % err)
> > +                    kdamonds.stop()
> > +                    exit(1)
> > +                # restore for comparison
> > +                ctx.pause = True
> 
> : If a kdamond contains multiple contexts, does this loop leave earlier contexts
> : paused in the kernel?
> : 
> : Since kd.commit() stages and commits the state of all contexts associated with
> : the kdamond, when kd.commit() is called for the second context, the first
> : context's local pause attribute is already back to True.
> : 
> : This would cause kd.commit() to write to sysfs and instruct the kernel to
> : pause the first context again. By the end of this loop, only the last context
> : in the kdamond would remain unpaused in the kernel.
> 
> No.  The pause field of the earlier context is set to False, so later
> kd.commit() will commit the False 'pause' again.  But this finds a good point.
> There is no reason to call kd.commit() for each context.  It is more efficient
> to be called for each kdamond., thouth currently we support only one context
> per kdamond.  I will update the code so, in the next spin.

I'm wrong, Sashiko is correct.  I missed the last 'ctx.pause = True' in the
loop.  I will fix this together, like below:

'''
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -201,16 +201,17 @@ def assert_ctx_committed(ctx, dump):

 def assert_ctxs_committed(kdamonds):
     ctxs_paused_for_dump = []
+    # pause for safe state dumping
     for kd in kdamonds.kdamonds:
         for ctx in kd.contexts:
             if ctx.pause is False:
                 ctx.pause = True
-                err = kd.commit()
-                if err is not None:
-                    print('pause fail (%s)' % err)
-                    kdamonds.stop()
-                    exit(1)
                 ctxs_paused_for_dump.append(ctx)
+        err = kd.commit()
+        if err is not None:
+            print('pause fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)

     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
     if err is not None:
@@ -218,17 +219,19 @@ def assert_ctxs_committed(kdamonds):
         kdamonds.stop()
         exit(1)

+    # resume contexts paused for safe state dumping
     for kd in kdamonds.kdamonds:
-        for ctx in kd.contexts:
-            if ctx in ctxs_paused_for_dump:
-                ctx.pause = False
-                err = kd.commit()
-                if err is not None:
-                    print('resume fail (%s)' % err)
-                    kdamonds.stop()
-                    exit(1)
-                # restore for comparison
-                ctx.pause = True
+        for ctx in ctxs_paused_for_dump:
+            ctx.pause = False
+        err = kd.commit()
+        if err is not None:
+            print('resume fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)
+
+    # restore for comparison
+    for ctx in ctxs_paused_for_dump:
+        ctx.pause = True

     ctxs = kdamonds.kdamonds[0].contexts
     dump = status['contexts']
'''


Thanks,
SJ

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