Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN

From: Steven Rostedt <rostedt@goodmis.org>
To: John Stultz <jstultz@google.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Yafang Shao <laoar.shao@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Network Development <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>,
	"linux-perf-use." <linux-perf-users@vger.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel test robot <oliver.sang@intel.com>,
	kbuild test robot <lkp@intel.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Michal Miroslaw <mirq-linux@rere.qmqm.pl>,
	Peter Zijlstra <peterz@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Kees Cook <keescook@chromium.org>, Petr Mladek <pmladek@suse.com>,
	Kajetan Puchalski <kajetan.puchalski@arm.com>,
	Lukasz Luba <lukasz.luba@arm.com>,
	Qais Yousef <qyousef@google.com>,
	Daniele Di Proietto <ddiproietto@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v2 7/7] tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN
Date: Sat, 11 Feb 2023 14:00:11 -0500	[thread overview]
Message-ID: <20230211140011.4f15a633@gandalf.local.home> (raw)
In-Reply-To: <20230208213343.40ee15a5@gandalf.local.home>

On Wed, 8 Feb 2023 21:33:43 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> OK, so it doesn't break perf, trace-cmd and rasdaemon, because the enum is
> only needed in the print_fmt part. It can handle it in the field portion.
> 
> That is:
> 
> 
> system: sched
> name: sched_switch
> ID: 285
> format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 
> 	field:char prev_comm[TASK_COMM_LEN];	offset:8;	size:16;	signed:0;
>                              ^^^^^^^^^^^^^^                          ^^
>                             is ignored                             is used
> 
> 
> 	field:pid_t prev_pid;	offset:24;	size:4;	signed:1;
> 	field:int prev_prio;	offset:28;	size:4;	signed:1;
> 	field:long prev_state;	offset:32;	size:8;	signed:1;
> 	field:char next_comm[TASK_COMM_LEN];	offset:40;	size:16;	signed:0;
> 	field:pid_t next_pid;	offset:56;	size:4;	signed:1;
> 	field:int next_prio;	offset:60;	size:4;	signed:1;
> 
> print fmt: "prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%s%s ==> next_comm=%s next_pid=%d next_prio=%d", REC->prev_comm, REC->prev_pid, REC->prev_prio, (REC->prev_state & ((((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) - 1)) ? __print_flags(REC->prev_state & ((((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) - 1), "|", { 0x00000001, "S" }, { 0x00000002, "D" }, { 0x00000004, "T" }, { 0x00000008, "t" }, { 0x00000010, "X" }, { 0x00000020, "Z" }, { 0x00000040, "P" }, { 0x00000080, "I" }) : "R", REC->prev_state & (((0x00000000 | 0x00000001 | 0x00000002 | 0x00000004 | 0x00000008 | 0x00000010 | 0x00000020 | 0x00000040) + 1) << 1) ? "+" : "", REC->next_comm, REC->next_pid, REC->next_prio
> 
>    ^^^^^^^
> 
> Is what requires the conversions. So I take that back. It only breaks
> perfetto, and that's because it writes its own parser and doesn't use
> libtraceevent.

Actually, there are cases that this needs to be a number, as b3bc8547d3be6
("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well") made
it update fields as well as the printk fmt.

I think because libtraceevent noticed that it was a "char" array, it just
defaults to "size". But this does have meaning for all other types, and I
can see other parsers requiring that.

-- Steve