Re: [PATCH] mk: using initial-exec model for thread local variable

From: Sachin Saxena <sachin.saxena@nxp.com>
To: Marvin Liu <yong.liu@intel.com>,
	"zhiyong.yang@intel.com" <zhiyong.yang@intel.com>,
	"thomas@monjalon.net" <thomas@monjalon.net>,
	"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [PATCH] mk: using initial-exec model for thread local	variable
Date: Thu, 5 Jul 2018 14:46:24 +0000	[thread overview]
Message-ID: <HE1PR0401MB1772274CE71E8959117A22B4E3400@HE1PR0401MB1772.eurprd04.prod.outlook.com> (raw)
In-Reply-To: <20180705141321.129989-1-yong.liu@intel.com>

> 
> When building share library, thread-local storage model will be changed to
> global-dynamic. It will add additional cost for reading thread local variable.
> On the other hand, dynamically load share library with static TLS will request
> additional DTV slot which is limited by loader. By now only librte_pmd_eal.so
> contain thread local variable. So that can make TLS model back to initial-exec
> like static library for better performance.
> 
> Signed-off-by: Marvin Liu <yong.liu@intel.com>
> 
> diff --git a/mk/toolchain/gcc/rte.vars.mk b/mk/toolchain/gcc/rte.vars.mk
> index 7e4531bab..19d5e11ef 100644
> --- a/mk/toolchain/gcc/rte.vars.mk
> +++ b/mk/toolchain/gcc/rte.vars.mk
> @@ -43,6 +43,13 @@ ifeq (,$(findstring -O0,$(EXTRA_CFLAGS)))  endif  endif
> 
> +# Initial execution TLS model has better performane compared to dynamic
> +# global. But this model require for addtional slot on DTV when dlopen
> +# object with thread local variable.
> +ifeq ($(CONFIG_RTE_BUILD_SHARED_LIB),y)
> +TOOLCHAIN_CFLAGS += -ftls-model=initial-exec endif
> +

[Sachin Saxena]   Using initial-exec model for shared object is not recommended. If you link a shared object containing IE-model, the object will have the DF_STATIC_TLS flag set. By the spec, this means that dlopen() might refuse to load it if TLS usage is greater than static TLS space.
This is what happening, when I tried to validate this change on ARM64 based NXP platform with VPP-dpdk solution. VPP initialization fails with following error:
  "load_one_plugin:145: /usr/lib/vpp_plugins/dpdk_plugin.so: cannot allocate memory in static TLS block"

Note that dpdk dpaa2 driver and VPP both uses TLS variables quite significantly. When forced to Initial-exec model in dpdk shared object, VPP static TLS space is getting exhausted and dlopen() returns error while trying to load the DPDK object.
For same reason, when we use "-fPIC" the default TLS model changed to "global-dynamics" from "Initial-exec".

In my opinion, this change should not be merged as it is breaking basic functionality.

>  WERROR_FLAGS := -W -Wall -Wstrict-prototypes -Wmissing-prototypes
> WERROR_FLAGS += -Wmissing-declarations -Wold-style-definition -Wpointer-
> arith  WERROR_FLAGS += -Wcast-align -Wnested-externs -Wcast-qual
> --
> 2.17.0