Subject: Re: [PATCH] tools/perf: Fix powerpc gap between kernel end and module start

From: Jiri Olsa <[email-masked]>
Date: Tue, 12 Jan 2021 10:38:11 +0100

On Mon, Dec 28, 2020 at 09:14:14PM -0500, Athira Rajeev wrote:

SNIP

> c000000002799370 b backtrace_flag
> c000000002799378 B radix_tree_node_cachep
> c000000002799380 B __bss_stop
> c0000000027a0000 B _end
> c008000003890000 t icmp_checkentry      [ip_tables]
> c008000003890038 t ipt_alloc_initial_table      [ip_tables]
> c008000003890468 T ipt_do_table [ip_tables]
> c008000003890de8 T ipt_unregister_table_pre_exit        [ip_tables]
> ...
> 
> Perf calls function symbols__fixup_end() which sets the end of symbol
> to 0xc008000003890000, which is the next address and this is the start
> address of first module (icmp_checkentry in above) which will make the
> huge symbol size of 0x80000010f0000.
> 
> After symbols__fixup_end:
> symbols__fixup_end: sym->name: _end, sym->start: 0xc0000000027a0000,
> sym->end: 0xc008000003890000
> 
> On powerpc, kernel text segment is located at 0xc000000000000000
> whereas the modules are located at very high memory addresses,
> 0xc00800000xxxxxxx. Since the gap between end of kernel text segment
> and beginning of first module's address is high, histogram allocation
> using calloc fails.
> 
> Fix this by detecting the kernel's last symbol and limiting
> the range of last kernel symbol to pagesize.
> 
> Signed-off-by: Athira Rajeev<[email-masked]>

I can't test, but since the same approach works for arm and s390,
this also looks ok

Acked-by: Jiri Olsa <[email-masked]>

thanks,
jirka

> ---
>  tools/perf/arch/powerpc/util/Build     |  1 +
>  tools/perf/arch/powerpc/util/machine.c | 24 ++++++++++++++++++++++++
>  2 files changed, 25 insertions(+)
>  create mode 100644 tools/perf/arch/powerpc/util/machine.c
> 
> diff --git a/tools/perf/arch/powerpc/util/Build b/tools/perf/arch/powerpc/util/Build
> index e86e210bf514..b7945e5a543b 100644
> --- a/tools/perf/arch/powerpc/util/Build
> +++ b/tools/perf/arch/powerpc/util/Build
> @@ -1,4 +1,5 @@
>  perf-y += header.o
> +perf-y += machine.o
>  perf-y += kvm-stat.o
>  perf-y += perf_regs.o
>  perf-y += mem-events.o
> diff --git a/tools/perf/arch/powerpc/util/machine.c b/tools/perf/arch/powerpc/util/machine.c
> new file mode 100644
> index 000000000000..c30e5cc88c16
> --- /dev/null
> +++ b/tools/perf/arch/powerpc/util/machine.c
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <stdio.h>
> +#include <string.h>
> +#include <internal/lib.h> // page_size
> +#include "debug.h"
> +#include "symbol.h"
> +
> +/* On powerpc kernel text segment start at memory addresses, 0xc000000000000000
> + * whereas the modules are located at very high memory addresses,
> + * for example 0xc00800000xxxxxxx. The gap between end of kernel text segment
> + * and beginning of first module's text segment is very high.
> + * Therefore do not fill this gap and do not assign it to the kernel dso map.
> + */
> +
> +void arch__symbols__fixup_end(struct symbol *p, struct symbol *c)
> +{
> +	if (strchr(p->name, '[') == NULL && strchr(c->name, '['))
> +		/* Limit the range of last kernel symbol */
> +		p->end += page_size;
> +	else
> +		p->end = c->start;
> +	pr_debug4("%s sym:%s end:%#lx\n", __func__, p->name, p->end);
> +}
> -- 
> 1.8.3.1
>