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SubjectRe: [PATCH v2] ftrace: simplify ftrace hash lookup code
On Mon, Sep 09, 2019 at 10:54:24AM -0400, Steven Rostedt wrote:
> On Mon, 9 Sep 2019 08:31:59 +0800
> Changbin Du <changbin.du@gmail.com> wrote:
>
> > Function ftrace_lookup_ip() will check empty hash table. So we don't
> > need extra check outside.
> >
> > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> >
> > ---
> > v2: fix incorrect code remove.
> > ---
> > kernel/trace/ftrace.c | 9 ++-------
> > 1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index f9821a3374e9..92aab854d3b1 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -1463,8 +1463,7 @@ static bool hash_contains_ip(unsigned long ip,
> > */
> > return (ftrace_hash_empty(hash->filter_hash) ||
> > __ftrace_lookup_ip(hash->filter_hash, ip)) &&
> > - (ftrace_hash_empty(hash->notrace_hash) ||
> > - !__ftrace_lookup_ip(hash->notrace_hash, ip));
> > + !ftrace_lookup_ip(hash->notrace_hash, ip);
>
> I don't care for this part. I've nacked this change in the past. Why?
> let's compare the changes:
>
> return (ftrace_hash_empty(hash->filter_hash) ||
> __ftrace_lookup_ip(hash->filter_hash, ip)) &&
> (ftrace_hash_empty(hash->notrace_hash) ||
> !__ftrace_lookup_ip(hash->notrace_hash, ip));
>
> vs:
>
> return (ftrace_hash_empty(hash->filter_hash) ||
> __ftrace_lookup_ip(hash->filter_hash, ip)) &&
> !ftrace_lookup_ip(hash->notrace_hash, ip);
>
> The issue I have with this is that it abstracts out the difference
> between the filter_hash and the notrace_hash. Sometimes open coded
> works better if it is compared to something that is similar.
>
> The current code I see:
>
> Return true if (filter_hash is empty or ip exists in filter_hash
> and notrace_hash is empty or it does not exist in notrace_hash
>
> With your update I see:
>
> Return true if filter_hash is empty or ip exists in filter_hash
> and ip does not exist in notrace_hash
>
> It makes it not easy to see if what happens if notrace_hash is empty
>
Yes, I agree with you entirly.

> Hmm, come to think of it, perhaps we should change ftrace_lookup_ip()
> to include what to do on empty.
>
> Maybe:
>
> bool ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
> {
> if (ftrace_hash_empty(hash))
> return empty_result;
>
> return __ftrace_lookup_ip(hash, ip);
> }
>
> Then we can change the above to:
>
> return ftrace_lookup_ip(hash->filter_hash, ip, true) &&
> !ftrace_lookup_ip(hash->notrace_hash, ip, false);
>
> That would probably work better.
>
> Want to send that update?
>
Yes, let me update it with your idea. Thanks!

> -- Steve
>
>
> > }
> >
> > /*
> > @@ -6036,11 +6035,7 @@ clear_func_from_hash(struct ftrace_init_func
> > *func, struct ftrace_hash *hash) {
> > struct ftrace_func_entry *entry;
> >
> > - if (ftrace_hash_empty(hash))
> > - return;
> > -
> > - entry = __ftrace_lookup_ip(hash, func->ip);
> > -
> > + entry = ftrace_lookup_ip(hash, func->ip);
> > /*
> > * Do not allow this rec to match again.
> > * Yeah, it may waste some memory, but will be removed
>

--
Cheers,
Changbin Du

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