Received-SPF: pass (google.com: domain of linux-kernel+bounces-68180-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Date: Fri, 16 Feb 2024 16:40:07 +0900
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Florent Revest
 <revest@chromium.org>, linux-trace-kernel@vger.kernel.org, LKML
 <linux-kernel@vger.kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>,
 bpf <bpf@vger.kernel.org>, Sven Schnelle <svens@linux.ibm.com>, Alexei
 Starovoitov <ast@kernel.org>, Jiri Olsa <jolsa@kernel.org>, Arnaldo
 Carvalho de Melo <acme@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
 Alan Maguire <alan.maguire@oracle.com>, Mark Rutland
 <mark.rutland@arm.com>, Peter Zijlstra <peterz@infradead.org>, Thomas
 Gleixner <tglx@linutronix.de>, Guo Ren <guoren@kernel.org>
Subject: Re: [PATCH v7 19/36] function_graph: Implement
 fgraph_reserve_data() and fgraph_retrieve_data()
Message-Id: <20240216164007.2de685ce5c78cee69e168601@kernel.org>
In-Reply-To: <20240214185407.767243b4@gandalf.local.home>
References: <170723204881.502590.11906735097521170661.stgit@devnote2>
	<170723226123.502590.4924916690354403889.stgit@devnote2>
	<20240214135958.23ed55e1@gandalf.local.home>
	<20240215084552.b72d6d22ce1b93bb8e04b70a@kernel.org>
	<20240214185407.767243b4@gandalf.local.home>
Precedence: bulk
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit

On Wed, 14 Feb 2024 18:54:07 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 15 Feb 2024 08:45:52 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > > Hmm, the above is a fast path. I wonder if we should add a patch to make that into:
> > > 
> > > 	if (unlikely(size_bytes & (sizeof(long) - 1)))
> > > 		data_size = DIV_ROUND_UP(size_bytes, sizeof(long));
> > > 	else
> > > 		data_size = size_bytes >> (sizeof(long) == 4 ? 2 : 3);
> > > 
> > > to keep from doing the division.  
> > 
> > OK, I thought DIV_ROUND_UP was not much cost. Since sizeof(long) is
> > fixed 4 or 8, so 
> > 
> > data_size = (size_bytes + sizeof(long) - 1) >> BITS_PER_LONG;
> > 
> > will this work?
> 
> No, because BITS_PER_LONG is 32 or 64 ;-)

Oops indeed.

> 
> But this should;
> 
> 	data_size = (size_bytes + sizeof(long) - 1) >> (sizeof(long) == 4 ? 2 : 3);
> 
> As sizeof(long) is a constant, that conditional expression will be hard
> coded into either 2 or 3 by the compiler.

Yeah. OK, let me update it.

Thank you,

> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>