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Message-ID: <Y7dBoII5kZnHGFdL@krava>
Date: Thu, 5 Jan 2023 22:31:12 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Zhen Lei <thunder.leizhen@...wei.com>
Cc: Petr Mladek <pmladek@...e.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Jiri Kosina <jikos@...nel.org>,
Miroslav Benes <mbenes@...e.cz>,
Joe Lawrence <joe.lawrence@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mark Rutland <mark.rutland@....com>, bpf@...r.kernel.org,
linux-trace-kernel@...r.kernel.org, live-patching@...r.kernel.org,
linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
linux-modules@...r.kernel.org
Subject: Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()
On Wed, Jan 04, 2023 at 05:25:08PM +0100, Petr Mladek wrote:
> On Fri 2022-12-30 19:27:28, Zhen Lei wrote:
> > Function __module_address() can quickly return the pointer of the module
> > to which an address belongs. We do not need to traverse the symbols of all
> > modules to check whether each address in addrs[] is the start address of
> > the corresponding symbol, because register_fprobe_ips() will do this check
> > later.
hum, for some reason I can see only replies to this patch and
not the actual patch.. I'll dig it out of the lore I guess
> >
> > Assuming that there are m modules, each module has n symbols on average,
> > and the number of addresses 'addrs_cnt' is abbreviated as K. Then the time
> > complexity of the original method is O(K * log(K)) + O(m * n * log(K)),
> > and the time complexity of current method is O(K * (log(m) + M)), M <= m.
> > (m * n * log(K)) / (K * m) ==> n / log2(K). Even if n is 10 and K is 128,
> > the ratio is still greater than 1. Therefore, the new method will
> > generally have better performance.
could you try to benchmark that? I tried something similar but was not
able to get better performance
I'll review and run my benchmark test tomorrow
thanks,
jirka
> >
> > Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> > ---
> > kernel/trace/bpf_trace.c | 101 ++++++++++++++++-----------------------
> > 1 file changed, 40 insertions(+), 61 deletions(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 5f3be4bc16403a5..0ff9037098bd241 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2684,69 +2684,55 @@ static void symbols_swap_r(void *a, void *b, int size, const void *priv)
> > }
> > }
> >
> > -struct module_addr_args {
> > - unsigned long *addrs;
> > - u32 addrs_cnt;
> > - struct module **mods;
> > - int mods_cnt;
> > - int mods_cap;
> > -};
> > -
> > -static int module_callback(void *data, const char *name,
> > - struct module *mod, unsigned long addr)
> > +static int get_modules_for_addrs(struct module ***out_mods, unsigned long *addrs, u32 addrs_cnt)
> > {
> > - struct module_addr_args *args = data;
> > - struct module **mods;
> > -
> > - /* We iterate all modules symbols and for each we:
> > - * - search for it in provided addresses array
> > - * - if found we check if we already have the module pointer stored
> > - * (we iterate modules sequentially, so we can check just the last
> > - * module pointer)
> > - * - take module reference and store it
> > - */
> > - if (!bsearch(&addr, args->addrs, args->addrs_cnt, sizeof(addr),
> > - bpf_kprobe_multi_addrs_cmp))
> > - return 0;
> > + int i, j, err;
> > + int mods_cnt = 0;
> > + int mods_cap = 0;
> > + struct module *mod;
> > + struct module **mods = NULL;
> >
> > - if (args->mods && args->mods[args->mods_cnt - 1] == mod)
> > - return 0;
> > + for (i = 0; i < addrs_cnt; i++) {
> > + mod = __module_address(addrs[i]);
>
> This must be called under module_mutex to make sure that the module
> would not disappear.
>
> > + if (!mod)
> > + continue;
> >
> > - if (args->mods_cnt == args->mods_cap) {
> > - args->mods_cap = max(16, args->mods_cap * 3 / 2);
> > - mods = krealloc_array(args->mods, args->mods_cap, sizeof(*mods), GFP_KERNEL);
> > - if (!mods)
> > - return -ENOMEM;
> > - args->mods = mods;
> > - }
> > + /* check if we already have the module pointer stored */
> > + for (j = 0; j < mods_cnt; j++) {
> > + if (mods[j] == mod)
> > + break;
> > + }
>
> This might get optimized like the original code.
>
> My understanding is that the addresses are sorted in "addrs" array.
> So, the address is either part of the last found module or it belongs
> to a completely new module.
>
> for (i = 0; i < addrs_cnt; i++) {
> /*
> * The adresses are sorted. The adress either belongs
> * to the last found module or a new one.
> *
> * This is safe because we already have reference
> * on the found modules.
> */
> if (mods_cnt && within_module(addrs[i], mods[mods_cnt - 1]))
> continue;
>
> mutex_lock(&module_mutex);
> mod = __module_address(addrs[i]);
> if (mod && !try_module_get(mod)) {
> mutex_unlock(&module_mutex);
> goto failed;
> }
> mutex_unlock(&module_mutex);
>
> /*
> * Nope when the address was not from a module.
> *
> * Is this correct? What if the module has gone in
> * the meantime? Anyway, the original code
> * worked this way.
> *
> * FIXME: I would personally make sure that it is part
> * of vmlinux or so.
> */
> if (!mod)
> continue;
>
> /* store the module into mods array */
> ...
>
>
>
>
> > + if (j < mods_cnt)
> > + continue;
> >
> > - if (!try_module_get(mod))
> > - return -EINVAL;
> > + if (mods_cnt == mods_cap) {
> > + struct module **new_mods;
> >
> > - args->mods[args->mods_cnt] = mod;
> > - args->mods_cnt++;
> > - return 0;
> > -}
> > + mods_cap = max(16, mods_cap * 3 / 2);
> > + new_mods = krealloc_array(mods, mods_cap, sizeof(*mods), GFP_KERNEL);
> > + if (!new_mods) {
> > + err = -ENOMEM;
> > + goto failed;
> > + }
> > + mods = new_mods;
> > + }
> >
> > -static int get_modules_for_addrs(struct module ***mods, unsigned long *addrs, u32 addrs_cnt)
> > -{
> > - struct module_addr_args args = {
> > - .addrs = addrs,
> > - .addrs_cnt = addrs_cnt,
> > - };
> > - int err;
> > + if (!try_module_get(mod)) {
> > + err = -EINVAL;
> > + goto failed;
> > + }
> >
> > - /* We return either err < 0 in case of error, ... */
> > - err = module_kallsyms_on_each_symbol(NULL, module_callback, &args);
> > - if (err) {
> > - kprobe_multi_put_modules(args.mods, args.mods_cnt);
> > - kfree(args.mods);
> > - return err;
> > + mods[mods_cnt] = mod;
> > + mods_cnt++;
> > }
> >
> > - /* or number of modules found if everything is ok. */
> > - *mods = args.mods;
> > - return args.mods_cnt;
> > + *out_mods = mods;
> > + return mods_cnt;
> > +
> > +failed:
> > + kprobe_multi_put_modules(mods, mods_cnt);
> > + kfree(mods);
> > + return err;
> > }
> >
> > int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>
> Otherwise, it looks good. IMHO, the new code looks more straightforward
> than the original one.
>
> Best Regards,
> Petr
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