linux-kernel - Re: [PATCH 2/3] riscv: Add support for perf registers sampling

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Date:   Fri, 12 Apr 2019 17:38:53 +0800
From:   Mao Han <han_mao@...ky.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, guoren@...nel.org
Subject: Re: [PATCH 2/3] riscv: Add support for perf registers sampling

On Thu, Apr 11, 2019 at 07:24:32AM -0700, Christoph Hellwig wrote:
> > --- /dev/null
> > +++ b/arch/riscv/kernel/perf_callchain.c
> > @@ -0,0 +1,122 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// Copyright (C) 2019 Hangzhou C-SKY Microsystems co.,ltd.
> 
> Please use normal /* */ Style comment for everything but the SPDX
> tags.
>
OK
 
> Please do early exists for all the error conditions.  Also no casts
> are needed for using ->fp as a scalar value, and we should probably
> just do a struct copy instead of copying both values individually.
> 
> The function should look something like:
> 
> static int unwind_frame_kernel(struct stackframe *frame)
> {
> 	if (!kstack_end((void *)frame->fp))
> 		return -EPERM;
> 	if ((frame->fp & 0x3 || frame->fp >= TASK_SIZE)
> 		return -EPERM;
> 
> 	*frame = *((struct stackframe *)frame->fp - 1);
> 	if (__kernel_text_address(frame->ra)) {
> 		int graph = 0;
> 
> 		frame->ra = ftrace_graph_ret_addr(NULL, &graph, frame->ra,
> 				NULL);
> 	}
> 	return 0;
> }
>
Thanks for suggestion. It looks much better with the modification.

> 
> Why not:
> 
> 	do {
> 		perf_callchain_store(entry, fr->ra);
> 	} while (unwind_frame_kernel(fr) >= 0);
> 
Yes, it's much simpler.

> > +/*
> > + * Get the return address for a single stackframe and return a pointer to the
> > + * next frame tail.
> > + */
> > +static unsigned long user_backtrace(struct perf_callchain_entry_ctx *entry,
> > +			unsigned long fp, unsigned long reg_ra)
> > +{
> > +	struct stackframe buftail;
> > +	unsigned long ra = 0;
> > +	unsigned long *user_frame_tail = (unsigned long *)(fp - sizeof(struct stackframe));
> 
> Overly long line.
Will fix the style.

> 
> > +	fp = user_backtrace(entry, fp, regs->ra);
> > +	while ((entry->nr < entry->max_stack) &&
> > +		fp && !((unsigned long)fp & 0x3))
> > +		fp = user_backtrace(entry, fp, 0);
> 
> Please don't indent the condition continuation and the loop body
> by the same amount.

Like this?
	while ((entry->nr < entry->max_stack) &&
					fp && !((unsigned long)fp & 0x3))
		fp = user_backtrace(entry, fp, 0);

Thanks,
Mao Han