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Message-ID: <20230814204723.0fd326c0@gandalf.local.home>
Date: Mon, 14 Aug 2023 20:47:23 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: David Laight <David.Laight@...LAB.COM>
Cc: Sven Schnelle <svens@...ux.ibm.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tracing/synthetic: use union instead of casts
On Mon, 14 Aug 2023 11:34:20 +0000
David Laight <David.Laight@...LAB.COM> wrote:
> > No need to create a structure around a single element union. Also, I would
> > like to name it for what it is for.
> >
> > union trace_synth_field {
> > u8 as_u8;
> > u16 as_u16;
> > u32 as_u32;
> > u64 as_u64;
> > struct trace_dynamic_info as_dynamic;
> > };
> >
> > Other than that, the patch looks good. Although I still need to test it.
>
> I was wondering if you need the u8 and u16 members at all?
> Can't the values just be treated as 32bit?
> Both char and short aren't really 'proper' arithmetic types.
Not sure what you mean by "arithmatic types".
If you do something like:
~# echo 's:skb u16 protocol; u64 delta;' >> /sys/kernel/tracing/dynamic_events
~# echo 'hist:keys=skbaddr:ts=common_timestamp.usecs' >> /sys/kernel/tracing/events/net/netif_receive_skb/trigger
~# echo 'hist:keys=skbaddr:delta=common_timestamp.usecs-$ts:onmatch(net.netif_receive_skb).trace(skb,protocol,$delta)' >> /sys/kernel/tracing/events/skb/kfree_skb/trigger
Where it records the protocol as u16, I'm guessing that if we didn't use
this union, it might break on BIG_ENDIAN machines.
For reference, the kfree_skb event looks like:
system: skb
name: kfree_skb
ID: 1735
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:void * skbaddr; offset:8; size:8; signed:0;
field:void * location; offset:16; size:8; signed:0;
field:unsigned short protocol; offset:24; size:2; signed:0;
field:enum skb_drop_reason reason; offset:28; size:4; signed:0;
-- Steve
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