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[2001:1ae9:1c2:4c00:726e:c10f:8833:ff22]) by smtp.gmail.com with ESMTPSA id p5-20020a05600c468500b004132ae838absm1458239wmo.43.2024.03.20.01.34.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 Mar 2024 01:34:12 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Wed, 20 Mar 2024 09:34:10 +0100 To: Masami Hiramatsu Cc: Andrii Nakryiko , bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org, kernel-team@meta.com, Peter Zijlstra Subject: Re: [PATCH bpf-next] bpf: avoid get_kernel_nofault() to fetch kprobe entry IP Message-ID: References: <20240319212013.1046779-1-andrii@kernel.org> <20240320124742.5652f47b8f6dfea24cf84ce9@kernel.org> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240320124742.5652f47b8f6dfea24cf84ce9@kernel.org> On Wed, Mar 20, 2024 at 12:47:42PM +0900, Masami Hiramatsu wrote: > On Tue, 19 Mar 2024 14:20:13 -0700 > Andrii Nakryiko wrote: > > > get_kernel_nofault() (or, rather, underlying copy_from_kernel_nofault()) > > is not free and it does pop up in performance profiles when > > kprobes are heavily utilized with CONFIG_X86_KERNEL_IBT=y config. > > > > Let's avoid using it if we know that fentry_ip - 4 can't cross page > > boundary. We do that by masking lowest 12 bits and checking if they are > > >= 4, in which case we can do direct memory read. > > > > Another benefit (and actually what caused a closer look at this part of > > code) is that now LBR record is (typically) not wasted on > > copy_from_kernel_nofault() call and code, which helps tools like > > retsnoop that grab LBR records from inside BPF code in kretprobes. I think this is nice improvement Acked-by: Jiri Olsa > > Hmm, we may better to have this function in kprobe side and > store a flag which such architecture dependent offset is added. > That is more natural. I like the idea of new flag saying the address was adjusted for endbr kprobe adjust the address in arch_adjust_kprobe_addr, it could be easily added in there and then we'd adjust the address in get_entry_ip accordingly jirka > > Thanks! > > > > > Cc: Masami Hiramatsu > > Cc: Peter Zijlstra > > Signed-off-by: Andrii Nakryiko > > --- > > kernel/trace/bpf_trace.c | 12 +++++++++--- > > 1 file changed, 9 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > > index 0a5c4efc73c3..f81adabda38c 100644 > > --- a/kernel/trace/bpf_trace.c > > +++ b/kernel/trace/bpf_trace.c > > @@ -1053,9 +1053,15 @@ static unsigned long get_entry_ip(unsigned long fentry_ip) > > { > > u32 instr; > > > > - /* Being extra safe in here in case entry ip is on the page-edge. */ > > - if (get_kernel_nofault(instr, (u32 *) fentry_ip - 1)) > > - return fentry_ip; > > + /* We want to be extra safe in case entry ip is on the page edge, > > + * but otherwise we need to avoid get_kernel_nofault()'s overhead. > > + */ > > + if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) { > > + if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE))) > > + return fentry_ip; > > + } else { > > + instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE); > > + } > > if (is_endbr(instr)) > > fentry_ip -= ENDBR_INSN_SIZE; > > return fentry_ip; > > -- > > 2.43.0 > > > > > -- > Masami Hiramatsu (Google) >