Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()

[Jiri Olsa <olsajiri@gmail.com>]

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@huawei.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